Abstract: M2 high speed steel has many sorts and shapes of eutectic carbides which are due to the variation in the conditions of solidification, alloying or heat treatment. This paper shows that colour metallographic tests can conveniently be used to clearly identify the kinds of the eutectic carbides in M2 high speed steel, and to study the reaction of decomposition of M₂C carbide at elevated temperatures, and also to distinguish the phase containing rich B from other phases.