We describe a new laboratory synthesis of the a -methyl ester sulfonates based on direct sulfonation of the methyl ester by SO3 introduced from the vapor phase. This was used to synthesize a chain deuterated sample of alpha C14MES, which was then used to measure the surface excess of aC14MES directly at the air/water interface over a wide range of concentration using neutron reflection. The adsorption isotherm could be fitted to an empirical equation close to a Langmuir isotherm and gave a limiting surface excess of (3.4 +/- 0.1) X 10(-6) mol m(-2) in the absence of added electrolyte. The neutron -measured surface excesses were combined with the integrated Gibbs equation to fit the variation in surface tension with concentration (sigma-1n C curve). The fit was exact provided that we used a prefactor consistent with the counterion at the surface being an impurity divalent ion, as has previously been found for sodium diethylhexylsulfosuccinate (aerosol OT or AOT) and various perfluorooctanoates. The critical micelle concentration (CMC) was determined from this fit to be 2.4 +/- 0.3 mM in the absence of electrolyte. In the presence of 100 mM NaC1, this contamination was suppressed and the sigma-ln C curve could be fitted using the integrated Gibbs equation with the expected prefactor of 1. The new data were used to reinterpret measurements by Danov et al. on an unpurified sample of alpha C14MES for which computer refinement was used to try to eliminate the effects of the impurities.