The interaction of the surfactant sodium dodecyl sulfate (SDS) with lipid monolayers of 1,2-dimyristoyl-d(54)-sn-glycero-3-phosphocholine (DMPC-d(54)) at the air/water interface was studied by infrared reflection absorption spectroscopy. To obtain separate information about the structural changes of both the lipid and the surfactant component during the interaction, isotopically labeled DMPC-d(54) was used. With increasing surfactant concentration, the slight shift of the CD2 stretching vibration of DMPC-d(54) toward lower wavenumber indicates a small increase of the lipid alkyl chain order due to the incorporation of the surfactant into the lipid monolayer. The DMPC-d(54) surface density stays the same. The frequency of the antisymmetric and symmetric CH2 stretching modes of the surfactant alkyl chain also shifts toward lower wavenumber with increasing amount of surfactant in the monolayer due to increased chain ordering. For the first time, we applied the isotopic substitution method for the direct calculation of surface partition coefficients describing the amount of incorporated SDS in the DMPC-d(54) monolayer. The partition coefficients were determined at different SDS concentrations and surface pressures and were compared to partition coefficients of the bilayer system, obtained under similar conditions by isothermal titration calorimetry. The best agreement between monolayer and bilayer data was obtained for a monolayer held at a pressure of 29.5-33.2 mN/m, which can be considered as the bilayer-monolayer equivalence pressure.