The present invention is predicated on the realisation that problems with the poor water solubility of arsenic trioxide and the extreme difficulty in dissolving arsenic trioxide in anything other than a very basic solution, could be overcome by forming a much more soluble diarsenic tetraoxide, including the compound NaHAs2O4, prior to its delivery to a patient. Pharmaceutical compositions with such compounds and their use in the treatment of cancers is disclosed.
