Delay times from teleseismic and local P wave arrivals are used to invert for a high-resolution three-dimensional velocity model beneath the northwest Pacific. The model shows high-velocity slabs with average velocity anomalies of the order of 3--4%. Assuming the positive velocity deviations in the subducting lithosphere are to first order due to a temperature anomaly, the results of a theoretical slab temperature profile based on the diffusion equation are converted to a synthetic slab velocity model. Temperature variations between the ambient mantle and the interior of the slab are converted to P wave velocity perturbations using dVp/dT≈4.8¿10-4 km s-1 ¿C-1. A nonlinear optimization scheme compares the tomograms obtained via tomography to the theoretically predicted models in order to determine the optimal values for slab thickness and mantle potential temperature. Using 1180¿100 ¿C as the potential temperature, thickness estimates of 88¿8 km, 85¿8 km, and 84¿8 km are obtained for the Izu-Bonin, Japan, and Kuril slabs, respectively. A correlation exists between slab thickness and age, which is strong if mantle temperature variations along the slab strike can be ruled out. In the process of estimating slab thickness the predicted slab velocity model is used as a filter to enhance the initial minimum-norm tomographic result. The initial tomogram is modified to closely resemble the synthetic slab tomogram by using only null-space components. The use of the null-space components guarantees that the enhanced solution will satisfy the original seismic delay times. The enhanced slab images show very continuous and narrow slabs compared to the initial tomographic results. ¿ 1999 American Geophysical Union