The phosphocholines 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC), 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and 1-oleoyl-2-stearoyl-sn-glycero-3-phosphocholine (OSPC) were exposed as monolayers on a Langmuir trough to O-3 in O-2 at concentrations from 0.3 to 30 ppm for reaction times of 10-30 min. The changes in the surface pressure-area isotherms after ozone exposure are shown to be sensitive indicators of reaction. The most dramatic changes were observed at the largest extents of reaction using a basic subphase, suggesting an acid was a major product of the O-3 reaction under these conditions. Consistent with this, in the monolayer POPC - O-3 reaction, the lipid acid 1-n-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PAPC) was isolated and identified as a major product of the reaction of 30 ppm O-3 for 30 min. As the concentration of O-3 and the reaction time were lowered, PAPC decreased while its aldehyde analog increased. At the lowest extent of reaction (0.3 ppm for 10 min), small amounts (similar to 5% yield) of the secondary ozonide were also observed. Possible mechanisms of reaction are discussed, as well as the implications of this work for the reactions of phosphocholines in pulmonary surfactant with inhaled oxidant air pollutants.