Abstract The effects of the position of sulfonate and of methyl groups on the aromatic ring of isomerically pure alkylxylenesulfonates on micellization and mixed micellization have been investigated using microcalorimetry and surface tension measurements. It was observed that the position of the sulfonate with respect to the alkyl chain is more critical than that of the methyl groups in determining the surfactant behavior in solution. Steric constraints to the packing of the molecules are proposed to be the main mechanism for the differences in the micellization of the surfactants. Calorimetric studies suggested that entropy was the main driving force for micellization. The mixed micellization of two structurally different alkylxylenesulfonates was found to be non-ideal. Regular solution theory adequately fits the data and microcalorimetry was used to verify the validity of the thermodynamic assumptions of the theory.