Dynamic light scattering is used as a sensitive probe of hybridization on DNA-functionalized colloidal gold nanoparticles. When a target DNA strand possesses an 8 base "dangling end", duplex formation on the surface of the nanoparticles leads to an increase in hydrodynamic radius. Duplex melting is manifested in a drop in hydrodynamic radius with increasing temperature, and the concentration dependence of the melting temperature provides a measure of the thermodynamics of binding. The hybridization thermodynamics are found to be significantly lower at higher hybridization densities than those previously reported for initial hybridization events. The pronounced deviation from Langmuir adsorption behavior is greater for longer duplexes, and it is, therefore, consistent with electrostatic repulsion between densely packed oligonucleotides. The results have implications for sensing and DNA-directed nanoparticle assembly.