The growth behavior of the primary alpha phase (Sr(NO3)(2)) and the secondary P phase (Sr(NO3)(2)(.)4H(2)O) in Sr(NO3)(2)-H2O peritectic system was observed in situ in a solution flow system. The two phases grew simultaneously below the peritectic temperature in the solution with the composition near the peritectic point. The beta phase grew much faster than the a phase due to the difference in both supersaturation and kinetic property for two phases. This resulted in the peritectic structure that the beta phase engulfed the alpha phase. Using Michelson interferometry, growth rate was measured 9 on the most widely developed faces, the (1 1 1) face of the a phase and the (1 1 0) face of the P phase. The P phase had the larger growth rate and step velocity and the smaller hillock slope than the alpha phase at a given supersaturation. The kinetic coefficient beta(s) and the edge free energy a, of steps were evaluated as follows: beta(s) = 5.1 x 10(-2) cm/s and alpha(s) = 3.6 mJ/m(2) for the alpha phase; beta(s) = 5.3 x 10(-1) cm/s and alpha(s) = 18 mJ/m(2) for the beta phase. This indicates that the step kinetics influences the growth of the two phases more significantly than that of the surface energy of step edge. (C) 2003 Elsevier Science B.V. All rights reserved.