The growth rate of ammonium pentaborate octahydrate has been measured as a function of supersaturation for various particle sizes, at different temperature range of 20-50degreesC in a laboratory-scale fluidized bed crystallizer. The values of mass transfer coefficient K-r reaction rate constant, k(r) and reaction order, r, were determined from the general mass transfer equation without assumption using an improved computer program which gives a high accuracy. The relative standard deviation between the overall growth rate data obtained from the general mass transfer equation by using kinetic parameters (kr, K and r) found in an improved program and those experimentally obtained do not exceed 0.8%. It was found that increasing the temperature and particle size cause an increase in the values of the kinetic parameters (K-g, k(r), K). Overall growth rate of ammonium pentaborate octahydrate is controlled by both diffusion and reaction steps at temperatures lower than 40degreesC, whereas it is completely controlled by diffusion at temperatures higher than 40degreesC. (C) 2003 Elsevier Science B.V. All rights reserved.