When butylamine (0.1-0.25 M) is added to a colloidal solution of CdSe nanoparticles (NPs) that are 1-2 nm in diameter, the band gap absorption changes from a broad, relatively weak band centered at 445 nm to a narrow, relatively strong absorption centered at 414 nm. The effect of concentration on the observed spectrum shows an isobestic point, suggesting an equilibrium between two species. Thermodynamic and kinetic studies of this process were carried out. The transition was found to be exothermic, with a size-dependent heat of reaction,that suggests that there is a direct proportionality between the NP size and the exothermicity of the interaction. The decay kinetics of the original peak were studied and compared to the rise kinetics of the 414 nm peak. The decay of the original NPs was biexponential, with a lifetime that depends on the NP size, concentration of reactants,. and the temperature. The rise in the 414 nm peak was found to be multiexponential, reflecting 2 transformations from the ensemble having different NP sizes. A mechanism is proposed for this interaction that depends on the binding of butylamine to the NP surface, followed by a release in energy. The nature of the new species absorbing at 414 nm is discussed.