Electronic energy transfer processes in chlorosomes isolated from the green sulphur bacterium Chlorobium tepidum and from the green filamentous bacterium Chloroflexus aurantiacus have been investigated. Steady-state fluorescence excitation spectra and time-resolved triplet-minus-singlet (TmS) spectra, recorded at ambient temperature and under non-reducing or reducing conditions, are reported. The carotenoid (Car) pigments in both species transfer their singlet excitation to bacteriochlorophyll c (BChl c) with an efficiency which is high (between 0.5 and 0.8) but smaller than unity; BChl c and bacteriochlorophyll a (BChl a) transfer their triplet excitation to the Car’s with nearly 100% efficiency. The lifetime of the Car triplet states is approximately 3 μs, appreciably shorter than that of the Car triplets in the light-harvesting complex II (LHCII) in green plants and in other antenna systems. In both types of chlorosomes the yield of BChl c triplets (as judged from the yield of the Car triplets) remains insensitive to the redox conditions. In notable contrast the yield of BChl c singlet emission falls, upon a change from reducing to non-reducing conditions, by factors of 4 and 35 in Cfx. aurantiacus and Cb. tepidum, respectively. It is possible to account for these observations if one postulates that the bulk of the BChl c triplets originate either from a large BChl c pool which is essentially non-fluorescent and non-responsive to changes in the redox conditions, or as a result of a process which quenches BChl c singlet excitation and becomes more efficient under non-reducing conditions. In chlorosomes from Cfx. aurantiacus whose Car content is lowered, by hexane extraction, to 10% of the original value, nearly one-third of the photogenerated BChl c triplets still end up on the residual Car pigments, which is taken as evidence of BChl c-to-BChl c migration of triplet excitation; the BChl c triplets which escape rapid static quenching contribute a depletion signal at the long-wavelength edge of the Q y absorption band, indicating the existence of at least two pools of BChl c.