DL-threo- and DL-erythro-β-Hydroxyisoleucine were prepared from cis- and trans-3-methylpent-2-enoic acids, respectively. The configuration of the erythro isomer was established from its synthesis by two independent routes. The diastereoisomers, and those of other β-hydroxy amino acids, can be distinguished by examination of their proton magnetic resonance spectra.