Central Fuel Research Institute, Dhanbad, Bihar Manuscript received 24 April 1978, revised 31 March 1980, accepted 31 March 1980 The catalytic vapour phase isomerisation of 1-methylnaphthalene to 2-methylnaphthalene and vice-versa was studied in the temperature range 420-620˚ and at varying liquid hourly space velocities on alumina-silica, alumina-chromia and alumina-boria of several molar compositions in flow systems using both fixed and fluidised bed conditions. The isomeric distribution of the products, as determined by a combination of spectroscopic and chromatographic methods, was found to be in conformity with the reported equilibrium values. Disproportionation, a competing paralled reaction during the isomerisation of methylbenzenes, did not occur in the case of methylnaphthalenes although some dealkylation to naphthalene was observed at higher reaction temperatures, especially under fixed-bed conditions. The isomerisation of 1-methylnaphthalene apparently obeyed first order rate law with an energy of activation of about 26 k cal/ mol, the value being close to those obtained with the xylenes. On the strength of the experimental evidence obtained, a mechanism involving the 1,2 shift of the methyl group through the formation of a localised complex intermediate has been postulated.