Aluminum oxide was deposited on the surface of 3mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP). The samples were annealed at temperatures from 1523 to 1773K. Diffusion profiles of Al in the form of mean concentration vs. depth in B-type kinetic region were investigated by secondary ion mass spectroscopy. The experimental results for the lattice diffusion (D-B) and grain boundary diffusion (D-GB) are as follows: where is the grain-boundary width and s is the segregation factor.