It is known that the glassy grain-boundary phase present in low-purity aluminas has two primary functions during direct microwave bonding. Firstly, it increases the dielectric loss of the host ceramic, allowing heating to occur; secondly, the bonding mechanism itself has been found to be based on viscous flow of the glassy grain-boundary phase. However, some evidence has also been found for the bonding of individual grains where they come into direct contact across the join line. To investigate the role of grain-boundary phases further, the microwave bonding of two different grades of silicon carbide and one grain of zirconia has been studied. A single-mode resonant cavity operating at 2450 MHz was used for both studies. The temperature and axial pressure were varied and the bonding time was kept to a minimum. Analysis of the resultant bonds indicated that both reaction-bonded silicon carbide and partially stabilized zirconia could be successfully joined using microwave energy with bonding times typically 10min or less. For reaction-bonded silicon carbide ceramics, the silicon grain-boundary phase softened at the bonding temperature, allowing the butting faces to be "glued" together. Unlike the glassy grain-boundary phase for alumina ceramics, the silicon phase did not allow grain motion but always formed a discrete and continuous layer at the interface, even under optimum joining conditions. The work with zirconia confirmed that it is possible to join ceramics without the presence of a substantial grain-boundary phase. The mechanism is thought to be either solid-state diffusion and/or grain-boundary sliding.