Abstract: Quantum yields for photochemical ring opening of six alkylcyclobutenes have been measured in hexane solution using 228-nm excitation, which selectively populates the lowest p,R(3s) excited singlet states of these molecules and has been shown previously to lead to ring opening with clean conrotatory stereochemistry. The compounds studied in this work - 1,2-dimethylcyclobutene (1), cis- and trans-1,2,3,4-tetramethylcyclobutene (cis- and trans-2), hexamethylcyclobutene (3), and cis- and trans-tricyclo[6.4.0.0^2,7]dodec-12-ene (cis- and trans-4) - were selected so as to span a broad range in molecular weight and as broad a range as possible in Arrhenius parameters for thermal (ground state) ring opening. RRKM calculations have been carried out to provide estimates of the rate constants for ground state ring opening of each of the compounds over a range of thermal energies from 20,000 to 44,000 cm^-1. These have been used to estimate upper limits for the quantum yields of ring opening via a hot ground state mechanism, assuming a value of kdeact = 10¹¹ s^-1 for the rate constant for collisional deactivation by the solvent, that internal conversion to the ground state from the lowest Rydberg state occurs with unit efficiency, and that ergodic behavior is followed. The calculated quantum yields are significantly lower than the experimental values in all cases but one (1). This suggests that the Rydberg-derived ring opening of alkylcyclobutenes is a true excited state process, and rules out the hot ground state mechanism for the reaction.