The inversion from R- to S-enantiomer that occurs for some arylpropionic acids may have both toxicological and therapeutic implications. To characterize some properties of this inversion, arylpropionyl-CoA thioester formation was studied in rat tissue homogenates and subcellular fractions for the enantiomers of fenoprofen, ibuprofen, and flurbiprofen. Thioesters were formed from (R)-fenoprofen (64%) and (R)-ibuprofen (33%) but not from the corresponding S-enantiomers or the enantiomers of flurbiprofen. This correlates with the extensive inversion of fenoprofen and ibuprofen and lack of inversion of flurbiprofen in vivo. Subcellular fractions from rat liver showed thioester formation to occur in mitochondria and microsomes but not cytosol. Once formed, the thioesters were readily racemized by whole rat liver homogenate, mitochondria, and cytosol, but only partially inverted (S:R = 0.3) in microsomes. Thioester formation from fenoprofen and ibuprofen was studied in tissue homogenate obtained from liver, diaphragm, kidney, lung, skeletal muscle, smooth muscle, fat, caecum, and intestines. The liver was at least 50-fold more efficient than the other tissues studied and would be expected to be a major organ of enantiomeric inversion. Our data support the hypothesis that R- to S-enantiomeric inversion of arylpropionic acids proceeds via the stereoselective formation of CoA thioesters followed by enzymatic racemization and hydrolysis of the thioesters to regenerate free acid.