Measurements of the size dependence of a solid-solid phase transition are presented. High-pressure x-ray diffraction and optical absorption are used to study the wurtzite to rock salt structural transformation in CdSe nanocrystals. These experiments show that both the thermodynamics and kinetics of this transformation are altered in finite size, as compared to bulk CdSe. An explanation of these results in the context of transformations in bulk systems is presented. Insight into the kinetics of transformations in both bulk and nanocrystal systems can be gained.