The production of sialon and sialon–boron nitride composites using oxidized (up to ~15 wt.% O) Si3N4–18 wt.% AlN composite powders resulting from plasma chemical synthesis is studied. Hot pressing of these powders to nonporous state at 1620–1775°C is accompanied by the formation of composites consisting mainly of a mixture of -sialon (z = 1.5–2) and -sialon. Boron nitride additions in an amount of up to 15 wt.% lead to the formation of composites that contain BN lamellar grains in the matrix of single-phase -sialon (z = 1.5–2). The mechanical and tribological characteristics of the composites with micron and submicron grains are examined. It is observed that the dependence of scratch hardness on contact strength is almost linear for the sialon ceramics and sialon–boron nitride composites. The material is removed during machining through intensive brittle fracture. The introduction of boron nitride into sialon ceramics in an amount of more than 5% is accompanied by approximately a tenfold increase in the material removal in machining, making possible turning operations.