Chlorosomes isolated from two types of green sulfur bacteria, Chlorobium tepidum which contains bacteriochlorophyll c (BChl c ) and the BChl e-containing Chlorobium phaeobacteroides, were subjected to alkaline treatment (pH 12.7 at 40°C for 20 min). This caused selective degradation of BChl a, whereas BChl c or e were not affected. Chlorobiumquinone in the chlorosomes was partially degraded by the alkaline treatment but menaquinone was unchanged. Fluorescence decay kinetics showed that alkaline treatment disrupted energy transfer from BChl c or e to BChl a under reducing conditions. However, this did not give rise to any substantial increase in the excited state lifetime of BChl e in C. phaeobacteroides chlorosomes, while for C. tepidum a decrease in the BChl c lifetime was found. The steady-state fluorescence of chlorosomes is highly dependent on the redox potential such that emission is quenched in oxidizing environments. Alkaline treatment diminished this quenching effect and caused a doubling in the BChl c or e emission intensity under aerobic conditions. Single-photon timing experiments confirmed that alkaline treatment inhibits the energy trapping process operative under aerobic conditions. These effects of alkaline treatment on the fluorescence intensity and decay kinetics are likely to be related to the depletion in BChl a or in chlorobiumquinone or a combination of these.