diff --git a/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp b/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp
--- a/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp
+++ b/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp
@@ -21,6 +21,10 @@
 // XFAIL: use_system_cxx_lib && x86_64-apple-macosx10.10
 // XFAIL: use_system_cxx_lib && x86_64-apple-macosx10.9
 
+// Libcxx when built for z/OS doesn't contain the aligned allocation funcitons,
+// nor does the dynamic library shipped with z/OS.
+// XFAIL: s390-zos
+
 // AppleClang < 10 incorrectly warns that aligned allocation is not supported
 // even when it is supported.
 // UNSUPPORTED: apple-clang-9
diff --git a/libcxx/test/libcxx/language.support/support.dynamic/new_faligned_allocation.pass.cpp b/libcxx/test/libcxx/language.support/support.dynamic/new_faligned_allocation.pass.cpp
--- a/libcxx/test/libcxx/language.support/support.dynamic/new_faligned_allocation.pass.cpp
+++ b/libcxx/test/libcxx/language.support/support.dynamic/new_faligned_allocation.pass.cpp
@@ -21,6 +21,10 @@
 // XFAIL: use_system_cxx_lib && x86_64-apple-macosx10.10
 // XFAIL: use_system_cxx_lib && x86_64-apple-macosx10.9
 
+// Libcxx when built for z/OS doesn't contain the aligned allocation funcitons,
+// nor does the dynamic library shipped with z/OS.
+// XFAIL: s390-zos
+
 // REQUIRES: -faligned-allocation
 // ADDITIONAL_COMPILE_FLAGS: -faligned-allocation