The influence of different additives and pH-values of the solution on the crystallization kinetics of Epsomite (MgSO(4)(.)7H(2)O) has been investigated in a fluidized bed crystallizer, For detection of changes in the metastable zone width. an ultrasonic technique has been used. In the experiment,, the supersaturation (0.2-2K), the impurity concentration (1-5 wt%) as well as the pH-value of the solution (2.5-8.6) ha e been varied. The growth rate of MgSO4. 7H(2)O crystals is related to the width of the metastable zone and supersaturation. The effects of the pH-value of the solution and the examined additives (KCl, K2SO4, MgCl2, and NaCl) can be divided into thermodynamic effects and kinetic effects, In the first case the impurities exert influences on the saturation and supersaturation limit whereas in the latter case the crystal growth is being suppressed. The thermodynamic properties of the concentration solutions are considered to explain the results. (C) 2002 Elsevier Science B,V, All rights reserved.