In situ C-13 MAS NMR spectroscopy under continuous-flow (CF) conditions was applied to study the mechanism of the methylation of phenol by methanol on acidic zeolite H - Y. Ex situ GC-MS was used as a complementary technique to confirm the nature of the chemical species remaining on the catalyst after the in situ MAS NMR investigations. Anisole and cresols were observed as the main reaction products in the initial step of the reaction. By use of the stopped-flow (SF) technique, the consecutive transformation of anisole to cresols was observed at the reaction temperature of 413 K indicating that anisole is an unstable primary product and an important intermediate.