Quantum yields for photochemical ring opening and cycloreversion in hydrocarbon solution have been determined for the direct photolysis (214 nm) of six 1,2-dimethylcyclobutene derivatives which contain methyl groups at C-3 and C-4 in numbers varying from zero to four. As the hydrogens on C-3/C-4 Of the parent compound (1,2-dimethylcyclobutene) are replaced with increasing numbers of methyl groups, the total quantum yield for ring opening increases to a maximum of similar to 0.3 and then decreases with further methyl substitution. The quantum yields for ring opening (phi(total)) of hexamethylcyclobutene and 1,2-dimethylcyclobutene are nearly the same, and the lowest in the series. The maximum occurs with trans-1,2,3,4-tetramethylcyclobutene; phi(total) for the cis-isomer is significantly lower, but both yield an approximate 1:1 mixture of formally allowed and forbidden diene isomers. A similar trend is observed in the relative quantum yields for ring opening and cycloreversion throughout the series, The results are interpreted in terms of a combination of bond strength and steric effects on the efficiency of the ring-opening process. Increasing methyl substitution causes an increase in phi(total) through the first three members of the series owing to progressive weakening of the C-3-C-4 bond. Compounds containing cis-dimethyl substitution exhibit substantially reduced quantum yields for ring opening, relative to what would be expected based on bond strength effects alone. This is proposed to be due to steric effects on the efficiency of the process, suggesting that the initial stages of the photochemical ring opening of cyclobutene involve disrotatory motions on the excited singlet state potential energy surface.