Excited 7-hydroxyquinoline embedded in a solid matrix of poly(2-hydroxyethyl methacrylate) undergoes a proton-relay reaction efficiently to form its keto tautomer. However, the reaction mechanism depends on the torsional conformation and the microscopic environment of the molecule at the moment of excitation. Whereas the bridged cis-enol form undergoes proton relay immediately on absorption of a photon to produce its tautomeric keto species, the unbridged cis form requires 120 ps for bridge formation via solvent reorganization prior to proton relay. Furthermore, the trans form needs 1000 ps for tautomerization because it requires an activated (11 kJ mol−1) torsional motion to change into its cis form prior to bridge formation and proton relay. Torsional motion rather than solvent reorganization determines the proton relay rate of the trans-form of the molecule.