Melting of diamond at high pressure and the properties of liquid carbon have been investigated with a first-principles molecular dynamics technique. The results indicate an increase of the diamond melting temperature with pressure, which is the opposite of the behavior of Si and Ge. This is contrary to long-held assumptions, but agrees with recent experiments. The structural and bonding properties of liquid carbon are found to vary strongly with pressure: At P ä 0 the system has an average coordination slightly smaller than that of graphite, with a considerable amount of sp sites, whereas at P > 1 Mbar its coordination is close to that of diamond. Both at low and high P, liquid carbon is found to be metallic. © 1991 IOP Publishing Ltd.