Methods of making an alpha-olefin sulfonic dimer acid (AOS dimer acid) are disclosed. In one method, an alpha-olefin is sulfonated, preferably with sulfur trioxide, to produce a mixture comprising an alpha-olefin sulfonic acid (AOS acid) and sulfur dioxide. This mixture is then heated while purging sulfur dioxide and hydrogen sulfide from the reactor to produce an AOS dimer acid composition. In another method, the AOS acid mixture is treated to remove sulfur dioxide and is then heated to produce an AOS dimer acid composition. With either method, the resulting AOS dimer acid composition has at least a 30% decrease in the level of elemental sulfur when compared with that of an AOS dimer acid composition prepared by a similar process in the absence of any active removal of sulfur dioxide or hydrogen sulfide. AOS dimer acid compositions and salts of the AOS dimer acids, which are useful surfactants for oilfield and other applications, are also disclosed.