Abstract—–Quantum efficiencies have been measured for the photoisomerization of four stereoisomers of retinal (all‐trans, 13‐cis, 11cis, and 9‐cis) in two solvents at different wavelengths of irradiation and at various temperatures. In heane at 25°C the quantum efficiencies for isomerization at 365 nm are: 9‐cistotrans, 0.5; 13‐cistotrans, 0.4; 11‐cistotrans, 0.2; all‐transtomonocisisomers, 0.2‐0.06, depending upon assumptions made regarding the stereo‐isomeric composition of the product. These values vary somewhat with the wavelength of the irradiating light.The quantum efficiency for the photoisomerization ofall‐transretinal in hexane decreases by a factor of 30 when the temperature is lowered from 25° to – 65°C; the activation energy for this photoisomerization is about 5 kcal/mole. The quantum efficiencies for the isomerization of themonocisisomers toall‐transretinal in hexane are virtually independent of temperature. In ethanol the rates of photoisomerization fromtranstocisorcistotransdepend only slightly on the temperature between 25° and – 65°C.The photosensitivities of the stereoisomers of retinal are of the same order of magnitude as those of the retinylidene chromophores of rhodopsin (11 ‐cis), metarhodopsin I (all‐trans), and isorhodopsin (9‐cis); but it is not yet possible to derive the photochemistry of rhodopsin uniquely