As part of the Bureau of Mines program to develop substitute or improved performance refractory materials for metallurgical applications, the Tuscaloosa Research Center has evaluated the effect of additives on sintering and strength properties of sialon (silicon aluminum oxynitride) compositions fabricated by a conventional powder metallurgy technique. Additions of y2o3, gd2o3, ceo2, mgo, zro2, zrsio4, and mgal2o4 to a sialon composition containing 70 pct b-si3n4 + 30 pct a-al2o3 increased sintered densities by as much as 50 pct over densities of undoped material. The studies also indicated that sintered densities reached a maximum between 20 and 30 mole-pct a-al2o3 when a series of sialon compositions of a-si3n4, having 6.7 to 57.9 pct a-al2o3, were doped with y2o3, gd2o3, and mgo. In studies to establish the effect of the crystalline phase of the silicon nitride and alumina used tests were made using a- and b-si3n4 and a- and v-al2o3 as starting materials. The tests indicated that when using an addition of 5 wt- pct y2o3, best results were obtained with a sialon composed of 74.4 Mole-pct a-si3o4 and 25.6 Mole-pct v-al2o3. In other tests made with this sialon composition modulus of rupture values of 28,120 and 29,500 psi at 25 deg. C, and 23,520 and 16,850 psi at 1,400 deg. C, were obtained when doping with 5 wt-pct zrsio4 and zro2, respectively.