The stericcourse of the gas-phase reaction of trans-[2H2]ethyleneoxide with HF, HCl, and HBr was investigated in order to test experimentally a mechanistic proposal based on ab initio calculations involving a concerted syn-opening mechanism. In contrast with this proposal the reactions with HCl and HBr take place entirely with anti-opening of the ring to give erythro-2-chloro- and 2-bromo[1,2-2H2]ethanol