Paraoxonase (PON-1) is a high-density lipoprotein (HDL)-bound enzyme with activity toward multiple substrates. It hydrolyzes organic phosphate and aromatic carboxylic acid esters. It also inhibits accumulation of oxidized phospholipids in plasma lipoproteins by a mechanism yet to be determined. Therefore, we subjected apolipoprotein A-I proteoliposomes containing either 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine or 1-palmitoyl-2-arachidonoyl-sn-glycero3-phosphocholine to oxidation by a peroxynitrite generator, SIN-1, in the presence and absence of purified PON-1. PON-1 modified the proportion of oxidation products without affecting the overall extent of PC oxidation. However, in the presence of PON-1, phosphatidylcholine isoprostanes were hydrolyzed to lysophosphatidylcholine. In addition, PON-1 hydrolyzed the phosphatidylcholine core aldehydes 1-palmitoyl-2(9-oxo)nonanoyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-(5-oxo)valeroyl-sn-glycero-3-phosphocholine to lysophosphatidylcholine. This hydrolysis was not affected by pefabloc, a serine esterase inhibitor. There was no detectable release of linoleate, arachidonate, or their hydroperoxy or hydroxy derivatives in the presence of PON-1. We conclude that PON-1 minimizes the accumulation of phosphatidylcholine oxidation products by the hydrolysis of phosphatidylcholine iso-PC derivatives including hydroperoxides, isoprostanes and core aldehydes. Recently, we reported that PON-1, in the absence of oxidant, hydrolyzed PC core aldehydes and produced lysophosphatidylcholine (lysoPC) (15). The present study demonstrates that PON-1 hydrolyzes PC isoprostanes, in addition to PC core aldehydes, to lysoPC. This hydrolysis was insensitive to pefabloc, a serine esterase inhibitor (16). Under these experimental conditions, PON-1 did not protect PLPC or PAPC against oxidation.