In reactions in which alkenes react in the presence of homogeneous Brønstedt acid catalysts, the protonation step is rate determining. Existing reaction rate correlations for protonation of butenes in sulfuric acid solutions are not consistent and limited to sulfuric acid concentrations below 80 wt %. The absorption of isobutene and trans-2-butene in sulfuric acid solutions was studied in the range of 30-98 wt %. Reevaluation of literature data, completed with new data from this study, revealed a set of kinetic rate data which coincide satisfactorily. Experiments and modeling of the absorption experiments showed a strong dependence of the reaction rate on local mixture composition through the activity coefficient of the protonating agent. This strong dependence prevents the occurrence of a diffusion-limited absorption regime. Application of the usually used criteria for operating in the kinetic regime can lead to erroneous results.