The addition of dicarboxylic acids to polyurethane adhesives, to improve their adhesion to vulcanized synthetic rubbers, may cause the degradation over time of the adhesives. To avoid the degradation of polyurethane adhesives, this study proposes a surface treatment on vulcanized synthetic rubbers with carboxylic acid solutions instead of adding them to the adhesives. In all the styrene-butadiene rubbers studied, a great improvement in adhesion to polyurethane adhesives has been obtained. The nature of the solvent used to disperse the fumaric acid over the surface, as well as the nature of the rubber, determined the effectiveness of the surface treatment. The use of fumaric acid solutions provided optimum results when the surface of the rubber was slightly roughened. The increase in the adhesion properties of styrene-butadiene rubbers, treated with fumaric acid, has been attributed to an increase in the degree of the surface roughness and to an enhancement in the surface energy (due to the elimination of abhesive substances on the surface of the rubber and to the formation of C-O groups by surface oxidation). The effectiveness of the surface treatment on rubbers with fumaric acid in the adhesion of styrene-butadiene rubbers has been compared with those produced by other common treatments, such as halogenation and roughening. In general, T-peel strengths obtained in the rubbers treated with fumaric acid were similar to those obtained through halogenation and were always higher than those obtained through roughening. Therefore, surface treatment of styrene-butadiene rubbers with fumaric acid can be considered as an alternative method to chemical treatments currently used, especially in the footwear industry.