diff -r a3e63cb21e20 -r 3dd7d21fb76b docs/src/modules.md
--- a/docs/src/modules.md	Tue May 29 11:05:12 2018 +0200
+++ b/docs/src/modules.md	Wed May 30 11:13:52 2018 +0200
@@ -455,6 +455,10 @@
 Be aware, that your destructor function should not free any memory, that is
 also managed by the pool.
 Otherwise you might be risking a double-free.
+More precisely, a destructor function set with `ucx_mempool_set_destr()` MUST
+NOT call `free()` on the specified pointer whereas a desructor function
+registered with `ucx_mempool_reg_destr()` MAY (and in most cases will) call
+`free()`.
 
 ## Properties