The adsorption of calf thymus DNA to 45 Angstrom nanoparticles of Cd(II)-rich CdS has been examined by photoluminescence spectroscopy as a function of temperature. The resulting van't Hoff plot suggests that the driving force for adsorption is entropy, and the enthalpic contribution to DNA-surface binding is slightly unfavorable. A likely source of the increase in entropy upon binding is release of solvent and/or counterions from the interface, analogous to what has been observed for nonspecific protein-DNA interactions. Reverse salt titrations suggest that counterion release is a substantial component of the nanoparticle-DNA interaction.