The product selectivity, reaction pathway, and the effects of, acid and base on the kinetics were studied for the methylation of phenol in supercritical methanol without catalyst at 380 degrees C and 0.237 g cm(-3). The reaction gave o-cresol as the major product and p-cresol and anisole in much smaller amounts, where the ortho/para ratio was above 12. The high ortho-selectivity suggests that the methylation proceeds locally around the phenolic OH group. Since no rearrangement was observed from anisole to o-cresol, the high ortho-selectivity was never ascribed to the rearrangement mechanism via anisole. Comparative experiments were carried out for other substituted benzenes to show that the phenolic OH group itself plays an essential role in the supercritical methylation. It was also found, in contrast to ordinary acid-catalyzed alkylations, that the supercritical methylation was retarded by acid and accelerated by base. The results were interpreted by a new reaction mechanism that the phenolic OH group acts as an acid catalyst in the supercritical methylation. The reaction mechanism was compared with that of noncatalytic isopropylation of phenol in supercritical water.