In solution phase synthesis of nanoparticles, processes such as coarsening and aggregation can compete with nucleation and growth in modifying the particle size distribution in the system. We show that coarsening of ZnO and TiO2 nanoparticles in solution follows the Lifshitz-Slyozov-Wagner rate law for diffusion controlled coarsening originally derived for colloidal systems with micrometer-sized particles, where the average particle size cubed is proportional to time. The rate constant for growth of ZnO in propanol is in the range 10(-4)-10(-2) nm3 x s(-1) and is dependent on the precursor anion and temperature. The coarsening of TiO2 nanoparticles from aqueous Ti(IV) alkoxide solutions is slower due to the low solubility of TiO2 with the rate constant in the range 10(-5)-10(-3) nm3 x s(-1) for temperatures between 150 degrees C and 220 degrees C. Epitaxial attachment of TiO2 particles becomes significant at higher temperatures and longer times. We show that the dominant parameters controlling the coarsening kinetics are solvent, precursor salt, and temperature.