A continuous process for the alkylation of an alkylatable hydrocarbon with an alkylating agent in the presence of an acid-type catalyst, including; contacting an alkylatable hydrocarbon with an alkylating agent in the presence of an acid-type catalyst at a temperature and pressure and for a time sufficient to alkylate the alkylatable hydrocarbon; separating the reaction effluent stream into an alkylate product phase and a catalyst phase containing catalyst-soluble oil; cooling the catalyst phase to a temperature essentially equal to the alkylation reaction temperature; recycling the cooled catalyst phase to the alkylation reaction; contacting a diolefinic hydrocarbon with an acid-type catalyst at a temperature and pressure sufficient to form additional catalyst-soluble oil and combining the additional catalyst-soluble oil with the circulating catalyst phase. In accordance with another aspect of the invention, a quantity selected from the amount of catalyst-soluble oil, water and/or the total of the amount of catalyst-soluble oil and water in the catalyst phase is controlled, within specific limits. A start-up procedure and apparatus for carrying out the process are also included.