The photostationary equilibrium between the Pr and Pfr forms of phytochrome [red and far red, respectively] shows a strong solvent deuterium isotope effect. Phytochrome transformation from the Pr to the Pfr form exhibits a small deuterium isotope effect, in Tris-D2O upon irradiation with red light, only after a photocycling of the phytochrome. In contrast, both the photoreversion and dark reversion of Pfr show an enhanced rate in D2O. In addition to the shift in the photostationary equilibrium in D2O, another pronounced effect of D2O on phytochrome is reflected in a significant enhancement of the fluorescence quantum yield of phytochrome (Pr). This result is interpreted in terms of the primary reaction involving an intramolecular proton transfer and its consequence in the phototransformation of phytochrome. A tyrosyl residue acts as a general acid catalyst in the Pr to Pfr phototransformation, which is slower in D2O than in H2O. The D2O solvent isotope effect on the photoreversion and dark reversion of Pfr is explained on the basis of acid catalysis, probably a specific acid catalysis by deuteronium ion.