Supercritical carbon dioxide is a promising solvent for the extraction and crystallization processes of natural compounds, especially thermolabil compounds. It is recognized as exhibiting powerful advantages over conventional solvents. However, so far a crucial point has been ignored, the possible oxidation of unsaturated compounds due to the presence of oxygen in high purity carbon dioxide used in these processes. The highest purity grade of commercial carbon dioxide is approximate to 2 mg/l STP. Taking into account that the solubility of most natural compounds is less than or equal to 0.1%, this means that the oxygen/compound concentration ratio would be high. To test this drawback, the extraction of p-carotene with supercritical carbon dioxide was studied. The p-carotene solubility was found to be 3.8 mg/kg at 30 MPa and 40 degreesC. Therefore, 263 kg of carbon dioxide would be needed to yield 1 g of beta -carotene. That meant a 9 mol oxygen/mol beta -carotene ratio. Results presented here suggest that beta -carotene was oxidized during the extraction process. (C) 2000 Elsevier Science B.V. All rights reserved.