p-Nitrophenyl methylphosphonate (PNPMP, 3) and p-nitrophenyl phenylphosphonate (PNPPP, 4) are rapidly hydrolyzed in acidic solutions (pH 3.5-6.0) at 37 degrees C by excess Zr4+, Ce4+, or Th4+ cations. Brij-35 micelles are employed in the Ce4+ and Th4+ cases. Second-order rate constants (M-1 s(-1)) with PNPMP are 115 (Zr4+), 53.9 (Ce4+), and 16.9 (Th4+), while for PNPPP they are, respectively, 76.8, 16.1, and 13.1. Relative to water (k(2) = 3.6 x 10(-11) M-1 s(-1) at 30 degrees C, pH 7.6), kinetic advantages with Zr4+ or. Ce4+ exceed 10(12).