On the basis of its spectroscopic and chemical properties, an unstable intermediate previously detected in the ozonolysis of trans-2,3-dichloro-2-butene (1) in inert solvents is reformulated as 1,1-dichloroethyl hydroperoxide (5). Ozonolysis of 1 in ethyl formate saturated with anhydrous HCl leads to high yields of this intermediate. 1,1-Dichloroethyl peroxide ion (10), rather than 5, is believed to be the precursor of acetyl 1,1-dichloroethyl peroxide (8), which is produced in higher yield on ozonolysis of 1 in the presence of tetraalkylammonium chloride.