Cubic structured CdS, CdSe, and CdTe, II-VI semiconductor nanoparticles have been synthesized using aqueous solution precipitation at room temperature. The "as-prepared" particles have a size of about 30 Å. Thermal annealing causes (a) an increase in particle size; (b) a structural transition from the cubic to the bulk, hexagonal structure for CdS and CdSe; and (c) no structural transition for CdTe. The unexpected cubic phase for small particles of CdS and CdSe may be due to either metastability or an equilibrium surface effect. The latter would imply a strong correlation between structure and the size and surface properties, and that there is a minimum size for hexagonal phase stability.