AbstractBacteriorhodopsin the protein of the purple membrane of Halobacteria, involves a retinylidene Schiff base as its photochemically active chromophore. Resonance Raman spectra, which selectively probe the retinylidene Schiff base chromophore, were obtained from aqueous suspensions of the purple membrane. In the unphotolysed chromophore Bacteriorhodopsin, the Schiff base nitrogen carries a proton whose reversible release during a photo‐induced cyclic process is essential for the ability of the purple membrane as a proton pump. It is expected that during this process the Schiff base group undergoes conformational and/or configurational changes, the elucidation of which is important for an understanding of the proton pump mechanism. We have looked for resonance Raman bands characteristic of the terminal group =C14HC15H=NH+‐ of the retinylidene Schiff base chromophore in BR570. Vibrational modes could be assigned on the basis of deuterium exchange at C15 and replacement of the Schiff baèe proton by a deuteron. Resonance Raman spectra of BR603, a chromophore with an additional double bond in the retinal ring (3‐dehydroretinal) but an identical terminal group, were also used for this analysis. Various types of hydrogen bending modes could be identified. From resonance Raman theory it was concluded that the intensities and frequencies of such modes reflect the conformation (bending and twisting angles) of the terminal group. A comparison of the resonance Raman spectra of BR570 (all‐trans) and the 13‐cis component BR548 from the dark‐adapted purple membrane revealed that the conformation of the protonated Schiff base is significantly different in these two chromophores. This is ascribed to variable interactions with the protein. The relation between resonance Raman spectroscopic data and the conformational structure of the terminal group is discussed on the basis of a simple model. From the results of this paper it is suggested that important structural information from resonance Raman spectroscopic data can also be obtained for retinylidene Schiff base chromophores in intermediates of the photochemical cycle.