An improved process for preparing olefin sulfonic acids and sulfonates by the reaction of sulfur trioxide and an olefin mixture having 14 to 16 carbon atoms per molecule which includes at least 30 percent by weight dimer olefins is disclosed. The olefin mixture and sulfur trioxide are mixed and reacted in a reaction zone of a continuous falling film reactor apparatus having a segregated cooling jacket, wherein the upper 1/8 to 1/3 of the reaction zone is operated and maintained at a constant temperature of between about 0.degree. to below about 27.degree. C and the lower 7/8 to 2/3 of the reaction zone is operated and maintained at a higher constant temperature of between about 25.degree. to about 40.degree. C. The acidic reactor effluent is then aged prior to neutralization. The improved process produces a resulting product having a low oil content and low color at increased production rates while greatly reducing the off gas plume for improved air pollution abatement.