The reactions of the cis- and trans-2-butenes photosensitized by Zn(3P1) have been studied. The main reaction was cis-trans isomerization. The photostationary [trans]/[cis] ratio was 1.5/1.0 and did not vary with pressure. This ratio is different from those obtained in the mercury- and cadmium-photosensitized reactions. The isomerization of 2-butenes photosensitized by Zn(3P1) could not be explained in terms of the simple mechanism proposed for the mercury- and cadmium-photosensitized reaction of 2-butenes. Possible mechanisms are discussed.