We have conducted diffusion measurements of radioactive Ag-110m tracer in single-crystal PbMgNbO3-PbTiO3 (PMN-PT) and in polycrystalline 50Pb(Ni1/3Nb2/3)O-3. 35PbTiO(3). 15PbZrO(3) (PNN-PT-PZ) piezoelectric ceramics. Both materials measured belong to the perovskite family. Diffusion in PMN-PT is characterized by an activation energy of 277 kJ/mol and pre-exponential factor of 0.0034 m(2)/S and compares well with cation diffusion in high-temperature superconducting YBa2Cu3O7-delta. Diffusion in polycrystalline PNN-PT-PZ, on the other hand, is many orders of magnitude faster and is attributed to grain boundaries. PNN-PT-PZ has a lower activation energy, 168 kJ/mol, and a combined pre-exponential factor (s delta (D-b)(o), where s is the segregation factor of silver, delta the thickness, and (D-b)(o) the pre-exponential factor for grain boundaries) of 3.7 x 10(-9) m(3)/s. The unusually large combined pre-exponential factor infers large segregation of silver at the grain boundaries and small solid solubility within the grains. It is possible, using a semiempirical model, to compute metal- (silver-) ceramic interface energies as a function of temperature, from which values of 90 kJ/mol and 0.9R are obtained for enthalpy and entropy, respectively, for grain-boundary segregation.