La(III), Pr(III), Nd(III), and Eu(III) (2 mM) can each combine with hydrogen peroxide to provide close to 9 orders of magnitude rate acceleration for hydrolyzing BNPP (bis(p-nitrophenyl) phosphate) at pH 7 and 25 degrees C. The rate of the reaction is second order each in [La(III)] and [H2O2], indicating that the active core 4 of the catalyst consists of two La(III) with two peroxides. The equilibrium constant for binding of BNPP to 4 (K-1 = (1.3 +/- 0.1) x 10(3) M(-1)) and the rate constant for hydrolysis of the bound phosphate (k(2) = (1.8 +/- 0.1) x 10(-2) s(-1)) have been determined. O-18 labeling studies reveal that the peroxide is a nucleophilic catalyst in cleaving the diester.