diff --git a/internal/core/source/source_test.go b/internal/core/source/source_test.go
index 1a21698..a7e2865 100644
--- a/internal/core/source/source_test.go
+++ b/internal/core/source/source_test.go
@@ -35,6 +35,11 @@ func TestValidateDocumentMissingFields(t *testing.T) {
 			mutate:  func(doc *SourceDocument) { doc.ID = " \t" },
 			wantErr: "source document id must not be empty",
 		},
+		{
+			name:    "id surrounding whitespace",
+			mutate:  func(doc *SourceDocument) { doc.ID = " source-1 " },
+			wantErr: "source document id \" source-1 \" must not contain leading or trailing whitespace",
+		},
 		{
 			name:    "kind",
 			mutate:  func(doc *SourceDocument) { doc.Kind = "" },
@@ -94,6 +99,11 @@ func TestValidateDocumentMissingUnitFields(t *testing.T) {
 			mutate:  func(doc *SourceDocument) { doc.Units[1].ID = "" },
 			wantErr: "source unit[1].id must not be empty",
 		},
+		{
+			name:    "id surrounding whitespace",
+			mutate:  func(doc *SourceDocument) { doc.Units[1].ID = " u2 " },
+			wantErr: "source unit[1].id \" u2 \" must not contain leading or trailing whitespace",
+		},
 		{
 			name:    "kind",
 			mutate:  func(doc *SourceDocument) { doc.Units[1].Kind = " " },
@@ -125,7 +135,7 @@ func TestValidateDocumentMissingUnitFields(t *testing.T) {
 
 func TestValidateDocumentDuplicateUnitIDs(t *testing.T) {
 	doc := validDocument()
-	doc.Units[1].ID = " u1 "
+	doc.Units[1].ID = "u1"
 
 	err := ValidateDocument(doc)
 
@@ -179,16 +189,31 @@ func TestValidateRefMissingUnitIDs(t *testing.T) {
 			ref:     SourceRef{StartUnitID: "u1", EndUnitID: "u2"},
 			wantErr: "source ref source_id must not be empty",
 		},
+		{
+			name:    "source id surrounding whitespace",
+			ref:     SourceRef{SourceID: " source-1 ", StartUnitID: "u1", EndUnitID: "u2"},
+			wantErr: "source ref source_id \" source-1 \" must not contain leading or trailing whitespace",
+		},
 		{
 			name:    "missing start id",
 			ref:     SourceRef{SourceID: "source-1", EndUnitID: "u2"},
 			wantErr: "source ref start_unit_id must not be empty",
 		},
+		{
+			name:    "start id surrounding whitespace",
+			ref:     SourceRef{SourceID: "source-1", StartUnitID: " u1 ", EndUnitID: "u2"},
+			wantErr: "source ref start_unit_id \" u1 \" must not contain leading or trailing whitespace",
+		},
 		{
 			name:    "missing end id",
 			ref:     SourceRef{SourceID: "source-1", StartUnitID: "u1"},
 			wantErr: "source ref end_unit_id must not be empty",
 		},
+		{
+			name:    "end id surrounding whitespace",
+			ref:     SourceRef{SourceID: "source-1", StartUnitID: "u1", EndUnitID: " u2 "},
+			wantErr: "source ref end_unit_id \" u2 \" must not contain leading or trailing whitespace",
+		},
 		{
 			name:    "unknown start id",
 			ref:     SourceRef{SourceID: "source-1", StartUnitID: "u9", EndUnitID: "u2"},
diff --git a/internal/core/source/validation.go b/internal/core/source/validation.go
index df38d32..dac150a 100644
--- a/internal/core/source/validation.go
+++ b/internal/core/source/validation.go
@@ -12,6 +12,9 @@ func ValidateDocument(doc *SourceDocument) error {
 	if isBlank(doc.ID) {
 		return fmt.Errorf("source document id must not be empty")
 	}
+	if hasSurroundingWhitespace(doc.ID) {
+		return fmt.Errorf("source document id %q must not contain leading or trailing whitespace", doc.ID)
+	}
 	if isBlank(doc.Kind) {
 		return fmt.Errorf("source document kind must not be empty")
 	}
@@ -27,20 +30,22 @@ func ValidateDocument(doc *SourceDocument) error {
 
 	seenUnitIDs := make(map[string]struct{}, len(doc.Units))
 	for i, unit := range doc.Units {
-		unitID := strings.TrimSpace(unit.ID)
-		if unitID == "" {
+		if isBlank(unit.ID) {
 			return fmt.Errorf("source unit[%d].id must not be empty", i)
 		}
+		if hasSurroundingWhitespace(unit.ID) {
+			return fmt.Errorf("source unit[%d].id %q must not contain leading or trailing whitespace", i, unit.ID)
+		}
 		if isBlank(unit.Kind) {
 			return fmt.Errorf("source unit[%d].kind must not be empty", i)
 		}
 		if isBlank(unit.Text) {
 			return fmt.Errorf("source unit[%d].text must not be empty", i)
 		}
-		if _, ok := seenUnitIDs[unitID]; ok {
-			return fmt.Errorf("source unit id %q is duplicated", unitID)
+		if _, ok := seenUnitIDs[unit.ID]; ok {
+			return fmt.Errorf("source unit id %q is duplicated", unit.ID)
 		}
-		seenUnitIDs[unitID] = struct{}{}
+		seenUnitIDs[unit.ID] = struct{}{}
 	}
 
 	return nil
@@ -53,12 +58,21 @@ func ValidateRef(doc *SourceDocument, ref SourceRef) error {
 	if isBlank(ref.SourceID) {
 		return fmt.Errorf("source ref source_id must not be empty")
 	}
+	if hasSurroundingWhitespace(ref.SourceID) {
+		return fmt.Errorf("source ref source_id %q must not contain leading or trailing whitespace", ref.SourceID)
+	}
 	if isBlank(ref.StartUnitID) {
 		return fmt.Errorf("source ref start_unit_id must not be empty")
 	}
+	if hasSurroundingWhitespace(ref.StartUnitID) {
+		return fmt.Errorf("source ref start_unit_id %q must not contain leading or trailing whitespace", ref.StartUnitID)
+	}
 	if isBlank(ref.EndUnitID) {
 		return fmt.Errorf("source ref end_unit_id must not be empty")
 	}
+	if hasSurroundingWhitespace(ref.EndUnitID) {
+		return fmt.Errorf("source ref end_unit_id %q must not contain leading or trailing whitespace", ref.EndUnitID)
+	}
 	if ref.SourceID != doc.ID {
 		return fmt.Errorf("source ref source_id %q does not match document id %q", ref.SourceID, doc.ID)
 	}
@@ -93,3 +107,7 @@ func UnitIndex(doc *SourceDocument, unitID string) (int, bool) {
 func isBlank(value string) bool {
 	return strings.TrimSpace(value) == ""
 }
+
+func hasSurroundingWhitespace(value string) bool {
+	return strings.TrimSpace(value) != value
+}