The influence of aggregation on triplet formation in the light- harvesting pigment-protein complex of photosystem II of green plants (LHCII) has been studied with time-resolved laser flash photolysis. The aggregation state of LHCII has been varied by changing the detergent concentration. The triplet yield increase upon disaggregation and follows the same dependence in the detergent concentration as the flourescence yield. The rate of constant intersystem crossing is not altered by disaggregation, and variations of the triplet yield appear to be due to aggregation-dependent quenching of singlet excited states. The efficiency of triplet transfer in LHCII aggregates from chlorophyll (Chl) to carotenoid (Car) is 92 ± 7% at room temperature and 82 ± 6% at 5 K, and does not change upon disaggregation. The Chl's that do not transfer their triplets to Car's seem to be bound to LHCII and are capable of transferring/accepting their singlet excitations to/from other Chl's. Two special contributions of Car triplets are observed: at 525 and 506 nm. Disaggregation of macroaggregates to small aggregates reduces by 10% the relative contribution of Car triplets absorbing at 525 nm. This effect most likely originates from a decreased efficiency of intertrimer Chl-to-Car triplet transfer. At the critical micelle concentration, at which small aggregates are disassembled into trimers, the interactions between Chl and Car are changed. At room temperature, this effect is much more pronounced than at 5 K.