Osmotic coefficients phi for aqueous solutions of sodium salts of n-butyl benzene sulfonate (NaNBBS), isobutyl benzene sulfonate (NaIBBS), tert-butyl benzene sulfonate (NaTBBS), n-propyl benzene sulfonate (NaPBS), ethyl benzene sulfonate (NaEBS), and p-toluene sulfonate (NaPTS) have been determined as a function of their molality m at 40 degrees C, 45 degrees C, and 50 degrees C by vapor pressure osmometry. Plots of phi versus 1/m show breaks yielding minimum hydrotropic molality (MHM) of the six hydrotropes. Burchfield and Woolley's model of micelle formation of ionic surfactants gives an apt representation of the experimental data.