diff --git a/src/corelib/text/qchar.cpp b/src/corelib/text/qchar.cpp
index 7b114e9723b..bf2a2a7e365 100644
--- a/src/corelib/text/qchar.cpp
+++ b/src/corelib/text/qchar.cpp
@@ -1588,6 +1588,11 @@ char32_t QChar::toLower(char32_t ucs4) noexcept
 
     Returns the uppercase equivalent if the character is lowercase or titlecase;
     otherwise returns the character itself.
+
+    \note This function also returns the original character in the rare case of
+    the uppercase form of the character requiring two or more characters.
+
+    \sa QString::toUpper()
 */
 
 /*!
@@ -1596,7 +1601,12 @@ char32_t QChar::toLower(char32_t ucs4) noexcept
     by \a ucs4 if the character is lowercase or titlecase; otherwise returns
     the character itself.
 
+    \note This function also returns the original character in the rare case of
+    the uppercase form of the character requiring two or more characters.
+
     \note Before Qt 6, this function took a \c uint argument and returned \c uint.
+
+    \sa QString::toUpper()
 */
 char32_t QChar::toUpper(char32_t ucs4) noexcept
 {
diff --git a/src/corelib/text/qlocale.cpp b/src/corelib/text/qlocale.cpp
index f9edd124d72..35e2707ac7e 100644
--- a/src/corelib/text/qlocale.cpp
+++ b/src/corelib/text/qlocale.cpp
@@ -3457,6 +3457,9 @@ Qt::LayoutDirection QLocale::textDirection() const
   Otherwise the conversion may be done in a platform-dependent manner,
   with QString::toUpper() as a generic fallback.
 
+  \note In some cases the uppercase form of a string may be longer than the
+  original.
+
   \sa QString::toUpper()
 */
 QString QLocale::toUpper(const QString &str) const
diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 8e4a625b1fd..cdb884b79f6 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -7247,7 +7247,10 @@ QString QString::toCaseFolded_helper(QString &str)
     \snippet qstring/main.cpp 81
 
     The case conversion will always happen in the 'C' locale. For
-    locale-dependent case folding use QLocale::toUpper()
+    locale-dependent case folding use QLocale::toUpper().
+
+    \note In some cases the uppercase form of a string may be longer than the
+    original.
 
     \sa toLower(), QLocale::toLower()
 */