Abstract: Process of preparing .beta.-isopropylnaphthalene substantially free of corresponding .alpha.-isomer, in yields amounting to 90% and more by a continuous two-stage process comprising (1) alkylating naphthalene with propylene at a temperature of about 150.degree.-280.degree. C. under a pressure of 5 to 30 atm in the presence of a phosphoric acid catalyst supported on a SiO.sub.2 carrier and using a mol ratio of 1/5 to 1/20 mol propylene to 1 mol naphthalene to isopropylate 45 to 65% of the naphthalene and form a mixture of .alpha.- and .beta.-isopropylnaphthalene and then without separating off the unreacted naphthalene, (2) heating the isomeric mixture from stage (1) in the presence of a phosphoric acid catalyst supported on SiO.sub.2, to a temperature of 180.degree.-280.degree. C. under an inert gas pressure of 5 to 30 atm until no further .beta.-isopropylnaphthalene is formed and recovering said .beta.-isopropylnaphthalene from the isomerization mixture.