Liposomes were used as a model membrane system to study the peroxidation of lipids by nitrogen dioxide (Nod. Small unilammelar liposomes of l-αdilinoleoyl phosphatidylcholine or l-αdilinolenoyl phosphatidylcholine were prepared by sonication. Exposure to NO2 was a bubbler apparatus at 1-10 ppm for 0-15 min. Initiation of phospholipid peroxidation was measured by diene conjugation at 232 nm for l-αdilinoleoyl phosphatidylcholine liposomes and at 235 nm for l-αdilinolenoyl phosphatidylcholine liposomes. The rate of peroxidation was consistent with a second-order rate model. The second-order rate constant for diene conjugation of l-αdilinoleoyl phosphatidylcholine was 1.5 × 10--3 M-1 s-1 and far l-αdilinoleoyl phosphatidylcholine was 3.1 × 10--3 M-1 s-1. The peroxidation reaction did not occur as a result of indirect reaction of the phospholipid with nitrite, and the rate constants are consistent with a mechanism of allylic hydrogen abstraction. © 1989 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.