We have combined neutron reflection measurements with isotopic substitution to determine the structure of a layer of sodium bis(2-ethyl-1-hexyl)sulfosuccinate (aerosol-OT or AOT) adsorbed at the air/solution interface. The widths of the distributions of the upper and lower halves of the molecule, as well as their positions in relation to the underlying water, have been measured at four concentrations varying from the critical micelle concentration (cmc) (2.5 x 10(-3) M) to CMC/300. Over this concentration range the coverage changes from 78 +/- 3 to 132 +/- 8 Angstrom(2)/molecule but the thickness of the surfactant layer does not change at all, unlike the behavior of all other soluble surfactants so far studied by neutron reflection. At the highest surface coverage we conclude that the butyl chains are strongly tilted away from the surface normal. At the lowest surface coverage the unexpectedly large width of the chain distribution is attributed to the molecule as a whole being tilted in such a way that the two ethyl hexanol fragments are on average at different levels in the interface. The only directly observable change in the layer is that the centers of the distributions of the two halves of the molecule move closer to the water as the surface concentration is reduced, suggesting that the molecules become more immersed in the water at low coverages.