Photolysis of bicyclo (1.1.0) butane by 185 or 214 nm radiation in hydrocarbon solution gave 1,3-butadiene and cyclobutene as primary photoproducts in a ratio of about 13:1. Isotopic labelling studies using bicyclobutane-2,2,4,4-d$\sb4$ as well as bicyclobutane-2-$\sp$C showed that the 1,3-butadiene was formed by a carbene extrusion pathway (cleavage of C$\sb1$-C$\sb3$ and C$\sb2$-C$\sb3$ followed by a hydrogen shift) two thirds of the time and by a direct rearrangement analogous to the thermal rearrangement the rest of the time. Cyclobutene formation could be by a hydrogen shift in a cyclobutanediyl intermediate, or via the carbene implicated in 1,3-butadiene formation. The exact electronic nature of the intermediate was not ascertained.

The operation of one additional photochemical pathway was discovered by irradiation of stereoisomers of the 2-methyl- and 2,4-dimethylbicyclobutanes. This pathway involved stereomutation of the substrate and was shown to occur through an intermediate cyclobutanediyl. The cyclobutene forming photoreaction became increasingly important as the degree of methyl substitution increased. The direct photorearrangement to dienes was partially stereospecific, and gave as a major product in each case the same diene stereoisomer as was produced in the thermal rearrangement. Partial stereospecificity was also seen for the carbene extrusion pathway.

Pyrolysis of the hitherto unknown endo,endo-2,4-dimethylbicyclobutane at about 250$\sp\circ$C gave primarily cis,trans-2,4-hexadiene. The degree of stereospecificity was similar to that observed in the pyrolysis of the other 2,4-dimethylbicyclobutanes. A new synthesis of the stereoisomers of the 2-methyl- and 2,4-dimethylbicyclobutanes was developed. Photolysis and thermolysis of 1-vinylbicyclobutane is also reported.