The kinetics of growth for the coating of particles in top-spraying fluidized bed systems is reported. The results indicated that only a small amount of particles that visited the spraying region are coated at a time. It was also revealed that different particle sizes are not equally coated during the process. In a polydispersed particle distribution, smaller particles were found to receive more coating than their larger counter-parts, This preferential coating, which was associated with a rapid decrease in the distribution variance, is more pronounced in the earlier parts of the process. When a narrower seed distribution was used, the preferential coating was reduced. A segregation factor, f(s) was introduced in the development of a growth kinetics model to represent the chance of each particle size visiting the coating region. The result for the distribution from the model clearly resembled the results obtained experimentally. For the top-spraying process, the segregation factor was found to be an exponentially decaying function of particle weight. For lactose particles coated with hydroxypropyl methyl cellulose. two different rates of growth were observed during the coating process.