The microstructure of a series of microwave sintered silicon nitride based ceramics have been assessed using a combination of optical microscopy/image analysis and analytical electron microscopy. Materials were studied as-received and after post-sinter hot-isostatic pressing. The grain size of microwave sintered materials was appreciably finer than conventionally processed ceramics of similar composition, although the mechanism involved is not clear. The as-received ceramics exhibited a reverse porosity gradient (with the highest porosity level at the surface) due to heat dissipation to the cooler surroundings during sintering. This also resulted in a small increase in the β′ grain aspect ratio close to the surface arising from an increase in the glass phase viscosity as the temperature decreases. Post-sinter HIPing of microwave sintered samples resulted in the elimination of most of the bulk porosity, but not near the surface. This is due to the reverse porosity gradient previously described, which leads to a transition from closed to open porosity with decreasing density near to the surface.