The effects of hydrodynamic conditions and surface quality on the growth rate of compressed polycrystalline disc of boric acid have been measured in a rotating disc crystallizer under well-established conditions of supersaturation. A new mass transfer equation based on an assumption that crystal growth depends mainly on two consecutive steps, diffusion and surface reaction, was proposed to describe the growth rate of crystals in rotating disc system. It was found that the mass transfer constant, K, and surface reaction constant, k(r), increased while overall growth rate order, g, and reaction rate order, r, decreased with increasing the solid-solution velocity and surface smoothness of disc. For both conditions, it was determined that the integration step mainly controls the growth fate of boric acid. The kinetic parameters (K, k(r) and r) were calculated for different conditions by trial and error method, under no assumption. (C) 2004 Elsevier B.V. All rights reserved.