The equilibrium boundary between diamond and graphite has been determined over the temperature interval 1100¿C-1625¿C. A piston-cylinder apparatus and a ''zero-friction'' cell were used. Pressures were computed by force/area measurements and are independent of any calibration points. Visual observations of diamond dissolution or diamond growth were used in fixing the position of the boundary. Our newly determined boundary has the equation P (kbar) =19.4+T (¿C)/40 kbar. Corrections for the effect of pressure on the emf of thermocouples have been made. Our newly determined boundary is almost coincident with the 1961 boundary published by the General Electric Company (Bundy et al., 1961).