Neutron reflection and surface tension measurements have been used to show that the surface properties of sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol-OT or AOT) at concentrations below the critical micelle concentration (cmc) are dominated by divalent ion impurities. Previous determinations of the area per molecule at the cmc using surface tension measurements and the Gibbs equation have given values of about 100 Angstrom(2) Or higher. Neutron reflection gives a lower value of 78+/-3 Angstrom(2). A similar Value can be obtained from surface tension measurements only in the presence of species which remove divalent ions from the solution. This suggests that the lower value is much closer to the correct value and that surface tension measurements on AOT will usually give misleading values of the coverage.