By using the closed-shell SCF MO method with the zero-differential overlap approximation for all valence electron systems previously presented by us, the electronic structures of trans- and cis- substituted ethylenes (treated substituent=CH3, Cl, Br, and I) are studied. The calculated values of the p and valence electron charges, and the total energies of these isomers are presented. The natures of the higher occupied MO’s, especially the lonepair MO’s, of chlorinated ethylenes are discussed in connection with the photoelectron observation. The calculated lowest p–p*singlet transition energies accord well with the observed values. As to the rotational energy changes, the calculated results show that, at the perpendicular configurations, the triplet states for all the treated compounds are lowest, while the intermediate states for the cis-transisomerizations of these compounds may be common to both the isomers.