The Fenna-Matthews-Olson (FMO) protein is a soluble light-harvesting, bacteriochlorophyll a (BChl a) containing antenna complex found in green sulfur bacteria. We have measured time-resolved fluorescence and transient absorption at variable laser intensities at 298 and 77 K using FMO protein from Chlorobaculum tepidum prepared in both oxidizing and reducing environments. Fitting of the spectroscopic data shows that high laser intensities (i.e., above 10(13) photons × cm(-2) delivered per laser pulse) distort the intrinsic decay processes in this complex. At high laser intensities, both oxidized and reduced FMO samples behave similarly, exhibiting high levels of singlet-singlet annihilation. At lower laser intensities, the reduced protein mainly displays a singlet excited state lifetime of 2 ns, although upon oxidation, a 60 ps lifetime dominates. We also demonstrate that the apparent quantum yield of singlet-triplet intersystem crossing in the reduced FMO complex is ∼11% in the most favorable low laser intensities, with this yield decreasing and the probability of singlet-singlet annihilation yield increasing as laser intensity increases. After correcting for stimulated emission effects in the experiments, the actual maximum triplet yield is calculated to be ∼27%. Experiments at 77 K demonstrate that BChl a triplet states in FMO are localized on pigments no. 4 or 3, the lowest energy pigments in the complex. This study allows for a discussion of how BChl triplets form and evolve on the picosecond-to-nanosecond time scale, as well as whether triplet conversion is a physiologically relevant process.