A material model has been presented, at the continuum level, for the representation of superplastic deformation coupled with microstructural evolution. The model presented enables the effects of the spatial variation of distributions of grain size to be predicted at the process level. The model has been tested under conditions of both homogeneous and inhomogeneous stress and strain by carrying out detailed comparison of predicted distributions of grain size and their evolutions with experimentally obtained data. Experimental measurements have shown the extent of the spatial variation of the distribution of grain size that exists in the titanium alloy, Ti-6Al-4V. It is shown that whilst not large, the variations in grain size distributions are sufficient to lead to the development of inhomogeneous deformation in test pieces, which ultimately result in localisation of strain and failure.