Abstract: An alkylation process that employs a refractive index analyzer to monitor, control, and/or determine acid catalyst strength before, during, or after the alkylation reaction. In a preferred embodiment, the invention relates to the alkylation of an olefinic feedstock with a sulfuric acid catalyst. The acid typically enters the alkylation reactor train at between from about 92 to about 98 weight percent strength. The concentration of acid is controlled and maintained by monitoring the refractive index of the acid in the product mixture comprising alkylate, mineral acid, water, and red oil. At least one online analyzer using a refractometer prism sensor producing real-time measurements of the refractive index of the solution may be compared to the results of manual laboratory tests on the acid strength of the catalyst using manual sample analyses or titration methods. Periodically, after calibration of the system, samples may be taken to verify the precision of the online analyzer, if desired.