A direct aqueous supercritical fluid extraction(SFE)system using carbon dioxide provides a sustainable means by which a vast range of industries may continue to depend on well established infrared(IR)techniques to determine oil-in-water.The SFE-IR method provides an environmentally friendly substitute for current national standard IR reference methods for measuring oil-in-water that rely on using increasingly restricted ozone depleting solvents whose manufacture is being phased out in accordance with international law.The SFE-IR analysis of a 500 mL water sample can be accomplished in 15 min.A rapid on-line SFE-IR calibration method has been implemented.With this calibration method,SFE-IR accuracy for determining diesel oil in 500 mL spiked water samples using single wave number measurement was 86.0%-98.8% with precision(RSD)ranging from 2.5%-7.0%.Using a general purpose calculation which involves measuring infrared absorbance values at three different wave numbers,SFE-IR method accuracy for determining diesel oil in 500 mL spiked water samples was 83.7%-92.2% with RSD 1.0%-9.3%.Data is presented that indicates current long established national standard IR reference methods involving three wave number calculations should be reviewed since,without careful consideration,the inclusion of calculated aromatic hydrocarbon species contributions to final oil-in-water concentration values may provide less accurate results.