In diffusion creep, the contribution of grain-boundary sliding to the overall strain o s can be evaluated in arbitrary polycrystals, if the angular distribution of grain boundaries is known. A o s value of 0.5 is obtained for two-dimensional equiaxed microstructures consisting of regular hexagonal grains, equiaxed grains grown from a Voronoi structure or grains having a circular distribution of grain-boundary angles. The o s value is also evaluated for uniaxially deformed 2D microstructures, both diffusionally and uniformly deformed. For the (...) former, the deformed microstructure is obtained by the simulation of microstructural evolution in polycrystals with straight grain boundaries. The o s value increases gradually with increasing or decreasing strain and is larger in the diffusionally deformed microstructures than in the uniformly deformed microstructures for a given grain aspect ratio. The o s value for three-dimensional polycrystalline microstructures is also obtained from an ellipsoidal distribution of grain-boundary angles. The resultant o s value is 0.60 for 3D equiaxed polycrystals and increases gradually with increasing strain. (shrink)