A study was made to understand the behaviour of glassy carbon under high pressure and temperature in the presence of a well-known catalyst solvent with a view to synthesize diamonds. Of the various catalysts available for diamond synthesis, an alloy of nickel and iron namely invar was used for the present study. Diamond synthesis experiments were carried out in the P-T region of 50-60 kb and 1300-1500degreesC. From a detailed study it was found that glassy carbon can be transformed into diamond at P > 55 kb and temperatures higher than 1200degreesC, although the conversion was low as compared to the yield of diamond synthesized using highly crystalline graphite. An interesting feature of this investigation is the presence of coesite phase in the reaction products synthesized under high P-T conditions whose origin appears to be related to the purity of the starting material. The results were also compared with some of the diamond synthesis experiments performed by using well-crystallized high-purity graphite. (C) 2003 Elsevier Science B.V. All rights reserved.