Steady-state and time-resolved absorption and fluorescence spectroscopic experiments have been carried out at room and cryogenic temperatures on aggregated and unaggregated monomeric and trimeric LHCII complexes isolated from spinach chloroplasts. Protein aggregation has been hypothesized to be one of the mechanistic factors controlling the dissipation of excess photo-excited state energy of chlorophyll during the process known as nonphotochemical quenching. The data obtained from the present experiments reveal the role of protein aggregation on the spectroscopic properties and dynamics of energy transfer and excited state deactivation of the protein-bound chlorophyll and carotenoid pigments.