Transformation of cis- and trans-2,3-dimethyloxiranes on a copper catalyst was studied at 363 K at various hydrogen pressures as well as in the presence of deuterium, in a recirculating reactor. The main reactions are hydrogenolysis, isomerization, and deoxygenation (2-butanol, 2-butanone, and 2-butenes are formed). In the course of transformation. the surface of the catalyst is oxidized via deoxygenation, leading to the generation of Lewis acid-base ion pairs. 2-Butanol and 2-butenes are formed on Cu(0) atoms while the formation of 2-butanone occurs mainly on copper ions. Differences between the reactivities of the oxirane isomers and the characteristic features of the mechanism of the three reactions observed are discussed in detail. Finally, experimental observations are compared with the transformation of cis- and trans-1,2-dimethylcyclopropanes under similar conditions.