A simple model of thermal dissociation and recovery of hydrogen-passivated silicon defects at the Si/SiO2 interface, such as P-b - centers, during vacuum thermal annealing has been suggested, his model considers reactions of hydrogen with defect states at the Si/SiO2 interface and diffusion of liberated atomic and molecular hydrogen in a silicon dioxide film. The rate constants were calculated in diffusion approximation. A good agreement was obtained between the experimental and numerical simulation results for oxides with different thickness (204-1024 Angstrom), grown, both. (111) and (100) samples and annealed in the temperature range (480-700 degrees C).