Liquid carbon dioxide was studied as a solvent for the isobutane-isobutylene alkylation. The acid catalysts in the reaction were anhydrous HF (AHF), pyridinium-poly(hydrogen fluoride) complex (PPHF), concentrated sulfuric acid and trifluoromethanesulfonic acid (TFSA). The effect of the acid-hydrocarbon volume ratio, temperature and residence time on the alkylate quality were studied over the temperature range of 50 greater than or equal to T greater than or equal to 0 degrees C. Carbon dioxide as a competing weak base decreases the acidity of the system which parallels the alkylate quality. In the case of HF and TFSA catalysts, solvent CO2 increased the octane number of the alkylate product (RON 95.6 for HF- and 88.0 for TFSA-catalyzed alkylation with CO2 solvent).