A detailed spectroscopic study of photoinduced and thermally activated proton-transfer processes for a series of different crystals of 2-(2,4-dinitrobenzyl)pyridine derivatives has been performed. The quantitative analysis of ground- and excited-state activation barriers and preexponential factors in deuterated and nondeuterated crystals shows clearly that the observed photochromism is linked to a proton-transfer process. Furthermore, it is clearly seen that the supramolecular environment of the transferred proton participates in the proton-transfer process. These supramolecular effects control the relative rates and efficiencies of the observed proton-transfer processes in both the ground and excited state, yielding, at room temperature, photoproducts having lifetimes ranging between hours and weeks. At least two proton-accepting groups may be active in the abstraction of the proton from its relatively stable benzylic position. Additionally, low-temperature measurements of proton-transfer processes show that tunneling processes prevail at temperatures below 100 K only in the excited state. No evidence for tunneling could be found for ground-state processes.