Process for the alkylation of aromatic hydrocarbons by contacting same with an olefin alkylating agent in a reaction zone maintained under conditions such that said alkylation is accomplished in the presence of a catalyst comprising a crystalline aluminosilicate zeolite, said zeolite having a silica to alumina ratio of at least about 12 and a constraint index, as hereinafter defined, within the approximate range of 1 to 12, said catalyst having been modified by prior thermal treatment to reduce the activity thereof, as determined by an alpha value, as described herein, to less than about 250 and preferably within the range of less than about 200 but greater than 10.