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The rapid and efficient methylation and extraction of organic acids from aqueous solution using supercritical carbon dioxide containing methyl iodide and tetrahexylammonium hydrogen sulfate was shown to be analogous to similar reactions in conventional liquid-liquid extractive alkylation systems. The test analytes 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were only extracted as their methyl esters by unmodified carbon dioxide at moderate temperature and pressure when methyl iodide was present in the extraction solvent Conversion to these derivatives from 1 ppm solutions was complete in 30 min with yields of 89% and 103% for 2,4-D and 2,4,5-T, respectively, and relative standard deviations better than 10%. The presence of methanol modifier in the supercritical carbon dioxide was deleterious to recovery of the methyl esters.A method in which the derivatizing reagent was continually supplied to the extraction vessel was found to be more demanding of reagents and equipment and gave less repeatable results than a simpler static reaction system. Supercritical fluid extraction (SFE) has been, for some years, an area of analytical chemistry with great potential which is yet to be fully realized. It is not only more rapid than traditional solvent extraction methods but has environmental and economic advantages over, for instance, the use of halogenated solvents under Soxhlet conditions. Carbon dioxide, by far the most