Carbamoylphosphonates (CPOs) have been identified as inhibitors of matrix metalloproteinases (MMPs) and as orally active, bioavailable, and safe antimetastatic agents. In this article, we focus on the direct antitumor activity of the CPOs. We discovered that CPOs also inhibit carbonic anhydrases (CAs), especially the IX and XII isoforms identified as cancer promoting factors. Thus, CPOs can be regarded as novel nontoxic drug candidates for tumor microenvironment targeted chemotherapy acting by two synergistic mechanisms, namely, inhibiting CAs and MMPs simultaneously. We have also demonstrated that the ionized CPO acid is unable to cross the cell membrane and thus limited to interact with the extracellular domains of isozymes CAIX and CAXII. Finally, applying CPOs against cancer cells in hypoxic conditions resulted in the dose dependent release of lactate dehydrogenase, confirming the direct interaction of the CPOs with the cancer related isozymes CAIX and XII and thereby promoting cellular damage.