Abstract: A new group of ceramic composite materials containing silicon oxynitride (Si.sub.2 N.sub.2 O, O'-phase) and zirconium oxide (ZrO.sub.2) can be prepared using a thermal reaction between zirconium silicate (ZrSiO.sub.4) and silicon nitride powder (Si.sub.3 N.sub.4) supplied completely or partly as silicon powder (Si) which is nitrided to Si.sub.3 N.sub.4 in a nitrogen-containing atmosphere. By adding aluminium oxide or the like, composite compositions with ZrO.sub.2, O'-phase and sialons (silicon aluminium oxynitrides) as main components can be obtained. In the method according to this invention, the raw powder material is prepared, e.g. by concurrent milling and blending to submicron particle size, formed by known technique into a compact, nitrided and sintered to the desired density in nitrogen-containing atmosphere.