The crystallization via incongruent melting and peritectic reaction was investigated in the Sr(NO3)(2)-H2O system. The dissolution processes of the secondary beta phase (Sr(NO3)(2)(.)4H(2)O) and the primary a phase (Sr(NO3)(2)) in water above and below the peritectic temperature T-p = 29.3degreesC were observed in situ, respectively, by optical microscopy. Around the beta phase dissolving above 61degreesC, alpha phase formed. With the depletion of the beta phase, the alpha phase stopped growing and dissolved. On the other hand, beta phase formed on the alpha phase dissolving below -13degreesC. After a while, ice crystals (gamma phase) formed and enclosed the P phase. The result shows that the incongruent melting and the peritectic reaction are the sequential processes of the dissolution of the metastable solid phase and the formation of the stable solid phase in the liquid. A possible driving force for the crystallization is originated from the difference in the solubility of the two solid phases at a given temperature. The formation of gamma phase on the alpha phase dissolving below 0degreesC was also observed, instead of the peritectic reaction. In this process, the a phase acted as the site for both melting and regrowth of the gamma phase. This resulted in a zonal structure of the gamma phase around the alpha phase. (C) 2003 Elsevier Science B.V. All rights reserved.