Crystallization of diamond from CO2 fluid was studied by using an equimolar mixture of C-12 graphite and C-13 graphitic carbon as a carbon source and PtO2 as an oxygen source under diamond-stable HP-HT condition at 7.7 GPa and 1500 degreesC. Before reaching the experimental condition, PtO2 was reduced by carbon, and CO2 was formed. Flaky graphite crystals less than 1 mum in size and octahedral diamond crystals about 10 gm. in size were obtained in the experiments held for 8 and 24 h, respectively. Both were solid solutions of C-12 and C-13, and the coexisting fluid was CO2 composed of (CO2)-C-12 and (CO2)-C-13. This means that starting graphitic carbon atoms of C-12 and C-13 Were mixed by the dissolution into CO2 fluid, and solid solution of graphite was first crystallized followed by that of diamond after a considerably long time. (C) 2002 Elsevier Science BY. All rights reserved.