Potassium pentaborate octahydrate (K2B10O16 - 8H(2)O) is used as an additive in industrial and automotive lubricants. While using in high-pressure lubricants, the particle size of crystals may be very important to obtain a stable dispersion. Therefore, the crystal growth kinetics must be known to obtain a desired particle size distribution. In the present work, the growth rate of potassium pentaborate octahydrate was investigated in a fluidized bed crystallizer under well-established conditions of supersaturation and fluidization. The seed size fractions of- 1000 + 850,-600 + 500, and 425 + 3 55 mu were used to determine the effect of initial crystal size on the growth rates. The influence of an external potential of I V on the growth rates of potassium pentaborate octahydrate crystals for the size range of - 1000 + 850 mu m was also determined. It was found that the overall growth rate of potassium pentaborate octahydrate changes linearly with supersaturation and does not depend on the initial size of seeds. Applying an external potential of I V also has no effect on the growth. The evaluation of growth rates in terms of isothermal effectiveness factors indicates an equal contribution of the mass transfer and the integration steps to the crystal growth of potassium pentaborate octahydrate. (c) 2005 Elsevier B.V. All rights reserved.