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Near-infrared excited Fourier-transform Raman spectra have been obtained from different spectral forms of Rhodobacter sulfidophilus light-harvesting II complexes. This complex, when isolated in lauryldimethylamine oxide, exists in a 805–828 nm form, which can be reversibly converted to the native 805–851 nm form upon addition of salt. The FT-Raman spectra predominantly show contributions of the carotenoid in the light-harvesting complex, with small but significant contributions of the bacteriochlorophylls excited in preresonance in the Q y transition. One strongly and one weakly interacting 2a acetyl C=O group as well as one moderately strong interacting and one non-interacting 9-keto C=O carbonyl modes of the bacteriochlorophylls can be discerned for the 805–828 nm form. Changes of relative band intensities caused by different resonance conditions for the different spectral forms lead to an assignment of the strongly interacting 2a acetyl C=O and the moderately strong interacting 9 keto C=O to bacteriochlorophylls organized in the 828 pigment moiety. Shifts of these bands to higher frequencies upon the salt-induced transition indicate a perturbation of the pigment-protein interaction, probably caused by a local protein conformational change.