Cyclopropane-d2 and 1,2-dimethylcyclopropane have been studied at 25°C and above. Cis-trans isomerization is apparently of general occurrence in Hg(3P1) photosensitization of substituted cyclopropanes. The most important primary process in the quenching of Hg atoms by cyclopropane-d2 seems to be the formation of triplet cyclopropane diradicals through which isomerization proceeds. Some exchange occurs at higher temperature. The Hg(3P1) photosensitized polymerization and exchange of D2 mixtures with cyclopropane also proceeds readily at elevated temperature (135°C).