The thermal isomerization of cis-3,4-dimethylcyclobutene in the vapor phase in the temperature range from 420 to 450°K was found to be a homogenous, unimolecular process. From the temperature dependence of the reaction, the first-order rate constant was observed to fit the equation k = 1013.88 exp(–34,000 ± 500/RT) sec-1. The single product that was obtained was cis,trans-2,4-hexadiene in agreement with the observations of Winter. The photosensitized decomposition of cis-3,4-dimethylcyclobutene using the Hg(3P1) atom as the sensitizer gave a mixture of isomers which consisted of 1,3-hexadiene, trans,trans-2,4-hexadiene, cis,trans-2,4-hexadiene, and a minor quantity of cis,cis-2,4-hexadiene. From quenching studies in which ether was used as the inert gas, it was inferred that three different excited states of cis-3,4-dimethylcyclobutene were involved in the photoreactions. The probable identities of these states are discussed in terms of the conservation of energy, orbital symmetry, and spin angular momentum. © 1969, American Chemical Society. All rights reserved.