The presence of graphite particles in gray cast iron commonly implies a good machinability, especially if compared to common steels. However, increasing industrial demands have resulted in a new interest for high-speed machining of these materials with higher performing tool materials than the traditionally used. Thus, the research presented in this paper focuses on evaluating the active tool wear mechanisms while high-speed machining EN-GJL-250 gray cast iron with pcBN tools with different composition, geometries and edge preparations. In addition to the expected flank and notch wear, formation of MnS, SiO2, and Al2O3 tool protection layers was found on the worn surfaces. Non-chamfered tools exhibited higher tendency towards notching. In general, the tools were found to have a rather small wear land but large edged radius indicating edge rounding being a more accurate wear criterion. (Less)