The microwave bonding of three alumina ceramics with different compositions has been investigated, without the use of interlayers, using a single-mode resonant cavity operating at 2450 MHz. The temperature and axial pressure were varied and the bonding time was kept to a minimum. For comparison, equivalent joins were produced using a standard diffusion bonding technique. Analysis of the resultant bonds indicated that, the lower the purity of the alumina, the more successful is the microwave bonding process. Whilst 99.8% alumina could not be heated to sufficiently high temperatures, the 94% and 85% aluminas could be joined successfully. The results indicated that a major function of the glassy grain-boundary phase was to increase the dielectric loss of the material. For the 85% alumina, bonding times were typically 10 min or less and total processing times were 30-45 min. This was much faster than could be satisfactorily achieved using diffusion bonding and resulted in significantly less deformation of the samples. The operational mechanisms, however, appeared to be almost identical between the two techniques and were based on viscous flow of the glassy grain-boundary phase. Provided that sufficient migration across the bond line of both gassy phase and grains occurred, then a fully homogeneous microstructure was obtained. In such cases, the mechanical strength of the bond could be at least as high as and often higher than that of the parent material, with the joined samples never breaking at the bond line during four-point bend tests.