Grain boundary (GB) wetting by a chemically compatible liquid in ceramic cuprates and its influence on transport properties of ceramics are considered, using model Bi2CuO4-Bi2O3 system as an example. It is shown that the GB wetting transition enhances significantly ionic conductivity and diffusional permeability of the ceramics. The experimental data on grain boundary wetting transition, electrical conductivity, diffusivity, and high temperature corrosion of copper are discussed. (C) 2005 Springer Science + Business Media, Inc.