PERSPECTIVEThe ozonolysis of an approximately one monolayer film of 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) on NaCl was followed in real time using diffuse reflection infrared Fourier transform spectrometry (DRIFTS) at 23 1C. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and Auger electron spectroscopy were used to confirm the identification of the products. Ozone concentrations ranged from 1.7 Â 10 12 to 7.0 Â 10 13 molecules cm À3 (70 ppb to 2.8 ppm). Upon exposure to O 3 , there was a loss of CQC accompanied by the formation of a strong band at B1110 cm À1 due to the formation of a stable secondary ozonide (1,2,4-trioxolane, SOZ). The yield of the SOZ was smaller when the reaction was carried out in the presence of water vapor at concentrations corresponding to relative humidities between 2 and 25%.