Due to their high kinetic inertness and consequently reduced side reactions with biomolecules, Pt^IV complexes are considered to define the future of anticancer platinum drugs. The aqueous stability of a series of biscarboxylato Pt^IV complexes was studied under physiologically relevant conditions. Unexpectedly and in contrast to the current chemical understanding, especially oxaliplatin and satraplatin complexes underwent fast hydrolysis in equatorial position (even in cell culture medium and serum). Notably, the resulting hydrolysis products strongly differ in their reduction kinetics, a crucial parameter for the activation of Pt^IV drugs, which also changes the anticancer potential of the compounds in cell culture. The discovery that intact Pt^IV complexes can hydrolyze at equatorial position contradicts the dogma on the general kinetic inertness of Pt^IV compounds and needs to be considered in the screening and design for novel platinum‐based anticancer drugs.