For samples conditioned under ambient air, the concentration of O2 at day 0 in the control and treated samples was about 15% which was below O2 content in air (20.96%) perhaps because of respiration of meat. Then the level of O2 decreased during chilled storage, from day 0 for HP-treated samples and day 14 for control samples, reaching in both samples levels close to 0% at day 21. This decrease of O2 concentra-tion during storage has also been observed by Andrés et al. (2006) in dry-cured Iberian ham packed under atmosphere composed of 30% CO2 with 5% O2. It can be ascribed to chemical reactions involving oxygen such as lipid oxidation or to microbial respiration of aerobic bacteria using available O2 in the headspace of packaging (Andrés et al., 2006). The increase of both aerobic mesophilic flora and lactic acid bacteria concomitantly to decrease of O2 content in non-pressurized samples could attest for the first hypothesis while for pressurized samples, the decrease of O2 content would be likely due to chemical reactions