Oxidation of polyethylene (PE) gives a mixture of degradation products. Carboxylic acids are among the major ones. They are believed to cause off-flavor in food and beverage packaging. Our previous research has shown that identification and use of indicator products gives earlier detection of oxidation in plastic materials. In the present study, the relation between off-flavor in water extracts of PE and each of the carboxylic acids in the homologous series from propanoic to nonanoic acid were investigated. Also, a comparison between carbonyl index in solid PE and each of the carboxylic acids in the homologous series from propanoic to decanoic acid were examined. The amount of the carboxylic acids was assessed by head-space solid-phase microextraction (HS-SPME) and gas chromatography-mass spectroscopy (GC-MS). It was found that the amount of butanoic and hexanoic acid correlated well with the rated off-flavor intensities in the water extracts. This confirmed earlier results on a linkage between these two carboxylic acids and highly rated off-flavor intensities. Moreover, a relation between the build-up of propanoic acid in the solid PE and carbonyl index was found. Hence, propanoic acid could be used as an indicator product for earlier oxidation detection in these PE materials. (c) 2006 Wiley Periodicals, Inc.