The forces between a glass sphere and a flat fused silica plate in aqueous surfactant solution were obtained by atomic force microscopy. The effect of interparticle separation on the adsorption of the cationic surfactant dodecyltrimethylammonium bromide to silica was determined in the presence of 20 mM KBr by application of a Maxwell relation. In comparison to results in the absence of added salt, the changes in adsorption all occur at smaller separations. This is consistent with electrostatic forces being the only significant long-range force. When the interacting surfaces are negatively charged, the surfactant adsorbs as the surfaces approach each other. When the interacting surfaces are positively charged, the surfactant usually desorbs as the surfaces approach each other. These patterns are entirely consistent with charge regulation of the interacting surfaces. The extent of adsorption, however, exceeds the amount required to obtain constant surface electrical potential. This shows that calculations of charge regulation for these surfactants should include short-ranged nonelectrostatic interactions between adsorbed molecules. At concentrations below the charge neutralization point, we also find that more surfactant is required to effect a similar amount of change in surface force when the salt is present. This is explained by competition between the surfactant and its co-ions for anionic surface groups.