ZnO samples were sintered in an overmoded 2.45 GHz microwave applicator. In-situ differential temperature measurements were made to allow comparison of surface and core temperatures during heating. At intermediate temperatures, near 600°C, the sample core was measured to be more than 250°C hotter than the sample surface. As the core temperature approached 1100°C, however, the difference between the surface and core temperatures diminished. Post-sintering scanning electron microscopy (SEM) showed spatial variations in the residual porosity which were consistent with the measured temperature differential. For samples sintered to intermediate temperatures, where large temperature differences persisted, there were significant gradients in the residual porosity. For samples sintered to higher temperatures, there was little residual porosity and no observable porosity gradient. Local density versus temperature behavior was obtained by correlating porosity levels measured from the micrographs with temperature measurements made during sintering. These data demonstrate a significantly lower activation energy for microwave sintering than for conventional sintering.