Hydrolyses in D2O (pD 1.7-3.1) of dimethyl (7), methyl phenyl (8), phenyl methyl (9), and diphenyl (10) phosphonoformate diesters are substantially accelerated by Ce(IV), Th(IV), Zr(IV), and Hf(IV) cations. Chemoselectivity is observed, whereby Zr(IV) and Hf(IV) principally direct P-OR hydrolysis, whereas Th(IV) and Ce(IV) mainly direct C-OR hydrolysis. Leaving group efficiency (OMe vs OPh) modulates the chemoselectivity. The metal cations also mediate further hydrolytic reactions of the initially produced phosphonoformate monoesters 14,15,18, and 20. The origins of the P-OR/C-OR selectivity are discussed in terms of the metal cation hydroxo species likely to be present in solution and the kinetics of the reactions.