The hydrolysis of adenosine triphosphate (ATP) [to adenosine diphosphate (ADP) and phosphate] at 40 °C by [Me3NH]6[H2MoV12O28(OH)12(MoVIO3)4]·2H2O (PM-17) as an important candidate for antitumor chemotherapy is investigated with the help of 31P NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and isothermal titration calorimetry (ITC). The ATP hydrolysis at pH 5 and 7.5 proceeds catalytically to yield ADP, HnPO4(3–n)–, adenosine monophosphate (AMP), and [(PO4)2Mo5O15]6–. AMP and [(PO4)2Mo5O15]6– result from the secondary step of the ATP hydrolysis: the former was produced by the hydrolysis of ADP (with the liberation of phosphate), and the latter by the condensation among phosphates and monomolybdates (derived from the partial release of the MoVIO3 moieties from the PM-17 anion). The enhancement of the ATP hydrolysis in the presence of K+ is discussed in terms of the electrostatic interaction of K+ at the Pγ phosphate in the negatively charged ATP 5′-triphosphates, which accompanies the coordination of PM-17 anions with a resultant conformational constraint of this phosphate. Values of thermodynamic parameters for the interaction between ATP and PM-17 at pH 7.5 suggest the K+-mediated assembly of PM-17 anions at the 5′-triphosphate chain (matrix effect), which leads to a decrease in the ATP hydrolysis owing to the depressed activation of the Pγ–O–Pβ linkage.