The effect of carbon content on the vacuum sintering characteristics of annealed and cold compacted water atomised type T1 high speed steel powders is reported. It was found that increasing carbon content from 0.8 to 1.4 wt-% resulted in significant improvements in sinterability; optimum sintering temperatures were lowered from 1320 to 1240 degrees C whereas the sintering window expanded from similar to 10 to similar to 40 K. Compositions with carbon contents outside this range were unsinterable owing to the formation of either undesirable M(6)C eutectic structures before attaining full density (0.6%C) or skeletal structures of an Fe-Cr rich phase on reaching full density (1.7%C). Sintering behaviour and microstructures are discussed with reference to phase diagrams. It is shown that optimum processing requires heating to temperatures within the austenite + carbide + liquid phase field with the width of the sintering window being determined by the temperature interval separating the phase boundaries which define this region. (C) 1995 The Institute of Materials.