As part of the Bureau of Mines program to develop substitutes for critical and strategic materials for metallurgical applications, the Tuscaloosa Research Center has evaluated the effects of additives and starting materials on the sintering and strength properties of sialon (silicon-aluminum-oxynitride) compositions fabricated by conventional pressing and sintering techniques. Sintering and mor (modulus of rupture) data were developed for sialon compositions produced from three components, si3n4, al2o3, and aln. When doped with y2o3 to aid in sintering, the highest mor values were obtained with compositions containing between 20 and 30 mole-pct (al2o3 + aln). The effect of the crystalline phase of the si3n4 and al2o2 used in the three-component system on mor was investigated. Alpha- si3n4 and y-al2o3 were found to be the best starting materials. A sialon composed of 67.4 Mole pct a-si3n4 plus 32.6 Mole-pct (y-al2o3 + aln), doped with 5 wt-pct y2o3 had mor values at 25 deg., 1,200 Deg., and 1,400 deg. C of 40,300, 40,300, and 31,100 psi, respectively. Samples of this composition doped with gd2o3, mgo, zro2, and zrsio4 also were evaluated. The zrsio4-doped sialon had higher mor values than gd2o3-, mgo-, and zro2-doped materials but lower mor values than the y2o3-doped sialon. The mor values of zrsio4-doped sialons were 25,000 and 25,900 psi, respectively, at 25 deg. and 1,400 deg c.