diff --git a/doc/ref/api-data.texi b/doc/ref/api-data.texi
index 41d4f73c9..ed14b2298 100644
--- a/doc/ref/api-data.texi
+++ b/doc/ref/api-data.texi
@@ -3183,9 +3183,10 @@ produce the corresponding string element.  The order in which
 @deffnx {C Function} scm_string_join (ls, delimiter, grammar)
 Append the string in the string list @var{ls}, using the string
 @var{delimiter} as a delimiter between the elements of @var{ls}.
-@var{grammar} is a symbol which specifies how the delimiter is
-placed between the strings, and defaults to the symbol
-@code{infix}.
+@var{delimiter} defaults to @w{@samp{ }}, that is, strings in @var{ls}
+are appended with the space character in between them.  @var{grammar} is
+a symbol which specifies how the delimiter is placed between the
+strings, and defaults to the symbol @code{infix}.
 
 @table @code
 @item infix
diff --git a/libguile/srfi-13.c b/libguile/srfi-13.c
index 97c372674..801591775 100644
--- a/libguile/srfi-13.c
+++ b/libguile/srfi-13.c
@@ -382,11 +382,12 @@ SCM_SYMBOL (scm_sym_prefix, "prefix");
 
 SCM_DEFINE (scm_string_join, "string-join", 1, 2, 0,
             (SCM ls, SCM delimiter, SCM grammar),
-	    "Append the string in the string list @var{ls}, using the string\n"
-	    "@var{delimiter} as a delimiter between the elements of @var{ls}.\n"
-	    "@var{grammar} is a symbol which specifies how the delimiter is\n"
-	    "placed between the strings, and defaults to the symbol\n"
-	    "@code{infix}.\n"
+            "Append the string in the string list @var{ls}, using the string\n"
+            "@var{delimiter} as a delimiter between the elements of @var{ls}.\n"
+            "@var{delimiter} defaults to @w{@samp{ }}, that is, strings in @var{ls}\n"
+            "are appended with the space character in between them.  @var{grammar} is\n"
+            "a symbol which specifies how the delimiter is placed between the\n"
+            "strings, and defaults to the symbol @code{infix}.\n"
 	    "\n"
 	    "@table @code\n"
 	    "@item infix\n"