The effect of different borax concentrations on the growth and dissolution rates of boric acid crystals were measured in a fluidized bed crystallizer under well-established conditions of supersaturation and undersaturation and fluidization. It was found that the presence of borax in boric-acid solution decreases the mass-transfer coefficient, k(d), the surface-reaction constant, k(r) and reaction order r pertaining to growth and dissolution rates of boric acid crystals. The effectiveness factors were estimated from the growth rate data to evaluate the relative magnitudes of the two resistances in series, diffusion and integration. The controlling mechanism is mainly by integration for the crystal growth of boric acid in the pure state and in the presence of borax in solution. The kinetic parameters (k(r), k(d), r) were determined by a new method which is called trial and error under no assumption. This method gives a high accuracy of determination of the mass-transfer coefficient. k(d). the surface-reaction constant, k(r) and surface-reaction order, r. The relative standard deviation between the equation R-g = k(r)(rho(alpha) - rho(eq)) - R-g(1 - w(alpha))/k(d))(r) and those experimentally obtained and represented by the equation R-g = k(g)(rho(alpha) -rho(eq))(g) do not exceed 0.013 for both the growth and dissolution regions. (C) 2002 Elsevier Science B.V. All rights reserved.