Quantum chemistry calculations have been used to study the conversion of spiropyran to merocyanine, both explicitly and with the model compounds pyran and nitrochromene. The minimum energy reaction path for the ring-opening reaction of pyran to form 2,4-pentadienal has been evaluated with Hartree-Fock calculations using the 3-21G and 6-31G(d) basis sets, as well as with semiempirical calculations. The reaction was found to follow a two-step mechanism, where first a conformer of the cis-2,4-pentadienal is formed, particularly a hydrogen-bonded seven-membered ring, and thereafter a rotation about a single bond to form one of the open-ring conformers of cis-2,4-pentadienal is indicated. Step one in this mechanism is the rate-limiting step, with a barrier of 33 kcal/mol at the Hartree-Fock 6-31G(d) level, corrected to 22 kcal/mol with Moller-Plesset second-order perturbation theory. This barrier is significantly higher than that reported previously using semiempirical calculations, and the discrepancy between the two studies is discussed. The energies of reaction for the ring-opening of nitrochromene and spiropyran have also been calculated by ah initio methods, and the effects of microsolvation and bulk solvation have been investigated for these systems. Finite field calculations were carried out to evaluate the polarizabilities and first hyperpolarizabilities and the effects of solvation on these properties.