On the basis of extensive quantum chemical calculations for 1,2 elimination of HX from C(2)H(5)X (X = F, Cl, Br, and I) we have obtained a strongly asymmetric transition state that differs greatly from previous models for this reaction. The effect of alpha-chloro/fluoro substitution was also investigated, specifically for HF and HCl elimination. We confirm the electron donor/acceptor property of these substituents but find the conventional explanation of their effect on the activation energy to be unsatisfactory.