Tin and molybdenum oxide catalytic systems were employed in transesterification and esterification reactions using methanol. In the presence of the molybdenum-based catalysts, low activity was observed in the methanolysis of soybean oil. However, high activity was attained in the case of ethyl acetate transesterification (conversion of 75% at 1 h, using SnMo50 as catalyst) and oleic or propionic acid esterification (conversion of 48% at 1 h, in the case of propionic acid, using SnMo50 as catalyst). This result may be associated with the formation of molybdenum carboxylates that are soluble in more polar media and are responsible for the catalytic activity. These results suggest that the conversions observed with these systems can be related to the formation of homogeneous catalytic complexes, promoting better diffusion of the substrate and the acid sites of these systems.