The pH dependence of the interfacial tension is an important factor in the behavior of sphingomyelin (SM) monolayers. We developed a theoretical model to describe this dependence in which the interfacial tension and molecular area contributions of each sphingomyelin form were additive and dependent on pH. The interfacial tension values and the molecular areas values for the SMH(+) and SMOH(-) forms of sphingomyelin were calculated and the proposed model was experimentally verified. The theoretical predictions agreed with the experimental results for pH values ranging from 2 to 12.