diff --git a/atom.go b/atom.go
index e9af0ac..24c163a 100644
--- a/atom.go
+++ b/atom.go
@@ -3,6 +3,7 @@ package atomfeed
 import (
 	"mime"
 	"net/url"
+	"regexp"
 	"strings"
 )
 
@@ -13,11 +14,20 @@ type (
 	URI          string
 )
 
-func isValidURL(testURL URI) bool {
-	_, err := url.ParseRequestURI(string(testURL))
+func isValidURL(uri URI) bool {
+	_, err := url.ParseRequestURI(string(uri))
 	return err == nil
 }
 
+func isValidURN(uri URI) bool {
+	pattern := `\A(?i:urn:(?!urn:)(?<nid>[a-z0-9][a-z0-9-]{1,31}):(?<nss>(?:[-a-z0-9()+,.:=@;$_!*'&~\/]|%[0-9a-f]{2})+)(?:\?\+(?<rcomponent>.*?))?(?:\?=(?<qcomponent>.*?))?(?:#(?<fcomponent>.*?))?)\z`
+	return regexp.MustCompile(pattern).MatchString(string(uri))
+}
+
+func isValidURI(uri URI) bool {
+	return isValidURL(uri) || isValidURN(uri)
+}
+
 func isCompositeMediaType(mediaType string) bool {
 	mediaType, _, err := mime.ParseMediaType(mediaType)
 	if err != nil {
diff --git a/category.go b/category.go
index acbc854..aa9f547 100644
--- a/category.go
+++ b/category.go
@@ -27,6 +27,12 @@ func (c *Category) Check() error {
 		return errors.New("term attribute of category empty")
 	}
 
+	if c.Scheme != "" {
+		if !isValidURI(c.Scheme) {
+			return fmt.Errorf("scheme attribute of category %v not correctly formatted", c.Scheme)
+		}
+	}
+
 	if c.Content == nil {
 		return errors.New("no content element of category")
 	} else {
diff --git a/outOfLineContent.go b/outOfLineContent.go
index 1a87068..b8ab7d7 100644
--- a/outOfLineContent.go
+++ b/outOfLineContent.go
@@ -22,7 +22,7 @@ func newOutOfLineContent(mediaType string, content any) (*OutOfLineContent, erro
 		return nil, fmt.Errorf("content type %T incompatible with out of line content", content)
 	}
 
-	if !isValidURL(content.(URI)) {
+	if !isValidURI(content.(URI)) {
 		return nil, errors.New("content not a valid uri")
 	}
 
diff --git a/person.go b/person.go
index 7314a7e..27818d9 100644
--- a/person.go
+++ b/person.go
@@ -29,7 +29,7 @@ func (p *Person) Check() error {
 	}
 
 	if p.URI != "" {
-		if !isValidURL(p.URI) {
+		if !isValidURI(p.URI) {
 			return fmt.Errorf("uri element of person %v not correctly formatted", p.Name)
 		}
 	}