This paper addresses the choice of spherical volume used in modelling MEG data recorded from temporal (auditory cortex) brain regions. We used MRI data sets to compute best-fitting spheres for the cranial cavity or inner skull table, and for the outer scalp surface. In addition, a 'generic' sphere was computed as the mean of the origin coordinates for the cranial spheres. The effects of these sphere choices were evaluated by estimating equivalent current dipoles as source generators for the M100 component of the auditory evoked field, and comparing the goodness-of-fit and 95% confidence volumes. Results indicate that for this area the cranial cavity is a better choice of surface to fit than the outer scalp. The 'generic' sphere results were also superior to those obtained using individual outer surfaces.