This invention provides novel crystalline forms of mono-HCl salts and a mono-HCl salt hydrate of JNJ-26481585, an inhibitor of histone deacetylases. The invention also relates to processes for production of these forms, to intermediates used in these processes, to pharmaceutical compositions comprising these forms, and to the use of these forms in medical treatment for instance as a medicine to inhibit proliferative conditions, such as cancer and leukemia.