The degradation of selected phenoxyalkanoic acid herbicides in soil under laboratory conditions was studied by using enantioselective high-resolution gas chroma tography/mass spectrometry (HRGC/MS). The compounds investigated were the chiral 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) and 2-(2,4-dichlorophenoxy)propionic acid (DCPP) and the achiral (4-chloro-2-methylphenoxy)acetic acid (MCPA), 2,4-(dichlorophenoxy)acetic acid (2,4-D), and dicamba, a benzoic acid derivative. Racemic and enantiopure MCPP and DCPP were incubated in separate experiments. In case of MCPP and DCPP, the herbicidally active R enantiomers were significantly slower degraded than the inactive S enantiomers. Incubation of enantiopure MCPP and DCPP revealed significant enantiomerization with formation of the R enantiomers from the S enantiomers, and vice-versa. Enantiomerization was found to be biologi cally mediated and was not observed in sterilized soil. Degradation followed initially approximate first-order kinetics with overall rates in the order of (S)-MCPP ≈ (S)-DCPP ≈ MCPA > 2,4-D > (R)-MCPP> dicamba > (R)-DCPP and with half-lives between 7 and 22 d, but with increased incubation time, the rates of degradation increased (half-lives, ≈4 d). The rates of enantiomerization were estimated to be ≈25−65% of the actual rates of decomposition. Mathematical models were developed and explained the general trends observed in the experimental data. It is anticipated that enantiomerization is an important process in the environmental fate and behaviour of these compounds and that the residues of MCPP and DCPP in environmental samples (soil, water) will not reflect the enantiomeric composition of the products used.