Magnesia, as a sintering additive, can be introduced into reaction-bonded silicon nitride (RBSN) via vapour-phase transport. The principal process variables were studied; a controlling factor was found to be the amount of silica on the internal surfaces of the ceramic. Through a controlled oxidation of the RBSN, the a mount of magnesia introduced to the compact was increased to approximate to 2 wt % allowing a post-sintered density of 93% theoretical to be achieved. Further increases in internal oxidation, and consequent magnesia uptake, were limited by the oxidation of Si3N4 whiskers on the surface of the RBSN and in its pore structure.