An integrated centroid path integral and free-energy perturbation-umbrella sampling (PI-FEP/UM) method for computing kinetic isotope effects (KIEs) for chemical reactions in solution and in enzymes is presented. The method is based on the bisection quantized classical path sampling in centroid path integral simulations to include nuclear quantum corrections in the classical potential of mean force. The required accuracy for computing kinetic isotope effects is achieved by coupled free-energy perturbation and umbrella sampling for reactions involving different isotopes. The use of FEP results in relatively small statistical uncertainties, whereas if kinetic isotope effects are computed directly by the difference in free energies obtained from the quantum mechanical potentials of mean force for different isotopes, the statistical errors are significantly greater. The PI-FEP/UM method is illustrated in two applications. The first reaction is the decarboxylation of N-methyl picolinate in water, and the primary 13 C and secondary 15 N KIEs have been determined. The second reaction is the proton-transfer reaction between nitroethane and acetate ions in water, and the total kinetic isotope effects were obtained. In both cases, the computational results are in accord with experimental results, and the findings provide further insight into the mechanism of these reactions in water.