Dissolution rates of two different glasses (soda-lime glass and fused quartz) and a natural opal (opal-CT) in distilled and deionized water and two concentrations of NaCI solution from 25°C to 75°C were measured by molybdate blue method and determined by initial rate method. The specific surface area of the samples were determined by N2 BET procedure. XRD patterns were obtained to check the crystallinity of these samples. Dissolution experiments show that soda-lime glass dissolves the fastest and opal-CT dissolve the most slowly. The dissolution rate of each sample is about one order of magnitude higher at 75°C than that at 2S°C. The calculated Ea for soda-lime glass is 32.7 kJ/mole, for fused quartz is 37.S kJ/mole and for opal-CT is 41.7 kJ/mole. / Master of Science