Abstract The methylation of decanoic acid during the supercritical fluid extraction process was investigated. It was found that the majority of the methylation occurred not during the extraction, but during the collection step, in the presence of methanol. Increasing dynamic extraction time (and hence collection time) from 0 to 60 min increased acid to ester conversions from 2% to 4%. The addition of HCl as a catalyst to the reaction increased the reaction rate 10-fold, and resulted in 88% conversion to the methyl ester within 30 min, without any detrimental effects on the chromatographic system. Higher collection temperatures appeared to increase conversion, but decreased methyl ester collection efficiency.