Light-induced isomerization of rhodopsin's retinal chromophore to the activating all-trans geometry initializes the formation of the active receptor state, Meta II. In the absence of peripheral regulatory proteins, the activity of Meta II is switched off spontaneously by two independent pathways: either by hydrolysis of the retinal Schiff base and dissociation of the light receptor into apoprotein opsin plus free retinal or by formation of Meta III, an inactive species with intact retinal protonated Schiff base absorbing at 470 nm. By FTIR spectroscopy on rhodopsin reconstituted with isotopically labeled chromophores in combination with quantum mechanical DFT calculations, we show that the deactivating step during formation of Meta III involves a thermal isomerization of the chromophore C[double bond]N, such that the chromophore in Meta III is all-trans-15-syn. This isomerization step is catalyzed by the protein environment and proceeds via Meta I, as suggested by its dependence on pH and on properties of the lipid/detergent environment of the protein. In the long term, Meta III decays likewise to opsin and free retinal by slow hydrolysis of the Schiff base.