The concentration-dependent spectroscopic characteristics of GaSe nanoparticles are interpreted in terms of the particles forming strongly interacting aggregates in high-concentration, room-temperature solutions. The high-concentration absorption spectra are semiquantitatively modeled in terms of the lowest two absorption bands of bulk GaSe, quantum confinement effects, and dipolar coupling between excited state monomers. The model predicts that the lowest energy absorption band shifts slightly to the red and sharpens, while the next band shifts to the blue as the concentration is increased. This correctly explains the observed absorption spectra and their reversible changes with concentrations. Static emission data and initial anisotropies of time-resolved emission may also be qualitatively understood on the basis of this simple model. From the model, the interparticle coupling, is estimated to be about -300 cm(-1). This is less than the energy differences between adjacent particles in the aggregates but much greater than what is observed between other types of semiconductor nanoparticles. Polydisperse samples have larger energy differences but comparable couplings between adjacent particles. As a result, the spectroscopic effects of aggregation are less pronounced. The nanoparticle aggregate spectra are reminiscent of J-aggregate spectra of organic dyes.