When model membranes are prepared by ultrasonic irradiation of polyunsaturated phospholipids, radical production can induce a partial degradation of the polyunsaturated fatty acyl chains and the formation of lipid hydroperoxides. A suitable buffer employed during liposome preparation, like Hepes or Tris, is able to exhibit a protective effect against lipid peroxidation. Hepes has been found to be the most effective: a 10 mM concentration provides a 70% protection after 30 min sonication. Tris, in the same conditions, exhibits a 50% protection. These findings may be explained on the basis of the rate constants of these organic buffers with hydroxyl radicals.