syn-Coplanar cleavage of hydrogen halide, esp. the syn -> trans process, was favored in the dehydrohalogenation of trans-1,2-dichlorocyclodecane and trans-1,2-dibromocyclodecane, and the anti elimination, which was most favored with acyclic compds., was retarded. The transition state for syn elimination was thought to be stabilized by redn. in transannular repulsive forces. The dehydrohalogenation rate depended only slightly on the solvent. [on SciFinder (R)]