Growth and dissolution rates of boric acid were measured in a laboratory-scale fluidized-bed crystallizer under well-established conditions of supersaturation, undersaturation, temperature and fluidization. It was found that the overall growth rate of boric acid is controlled by both diffusion and reaction steps. But reaction step is the main controller of the crystal growth of boric acid. It was also determined that the dissolution region of boric acid, apart from other known materials is controlled by diffusion and reaction steps. An improved computer program procedure was used to determine the effect of diffusion and reaction steps on the growth and dissolution rates of boric acid crystals. This simple program gives a high accuracy of determination of the mass transfer coefficient, k(d), the surface reaction constant, k(r) and the surface reaction order r. The relative standard deviation between the equation R-G = k(r)(Delta C - R-G/F-d)(r) and those experimentally obtained and represented by the equation R-G = K-G(Delta C)(g) do not exceed 0.08 % for both growth and dissolution region. (C) 2000 Published by Elsevier Science B.V. All rights reserved.