Photolysis of 3-(2,2-dimethylcyclopropyl)-3-chlorodiazirine (4) at 25-degrees-C in pentane affords 81% of the 1,2-C migration products 1-chloro-3,3-dimethylcyclobutene (5) and 2-chloro-3,3-dimethylcyclobutene (6) in a 4.8:1 distribution, as well as 19% of the fragmentation products, isobutene, and chloroacetylene. Experiments in the presence of the carbene trap trimethylethylene point to product formation from the electronically excited diazirine (4*), which affords 19-20% of fragmentation products, approximately 17% of 5 and 6 (3.8 : 1), and approximately 63% of 2,2-dimethylcyclopropylchlorocarbene(3). The carbene ring expands to 5 and 6 (5.2:1). Laser flash photolycic studies afford absolute rate constants for 3 --> 5 (1.3 x 10(6) S-1) and 3 --> 6 (2.5 X 10(5) S-1). The 5-fold preference of 3 for CH2 migration (to 5) over CMe2 migration (to 6) is attributed to differential steric affects. Ab initio calculations afford structures and energies for 3 and the CH2 and CMe2 transition state (8-CH2 and 8-CMe2). The calculations agree with the experimental findings in that DELTAG(double dagger) for CH2 Migration is found to be 1.4 kcal/mol less than DELTAG(double dagger) for CMe2 Migration, corresponding to a computed 10-fold kinetic preference for CH2 migration. The expected additional steric congestion is apparent in the 8-CMe2 transition state.