Three-compartment representation is applied in the modelling of particulate coating in a fluidized-bed granulator. The model based on the random surface renewal concept was employed to develop population balance equations for the coated particles. The model is used in assessing the effect of stagnant regions, large beds and unequal rate of transfer between domains on the coating distribution. It was found that the existence of stagnant regions within the bed will widen the coating distribution, while increasing the rate of transfer to the active spraying domain relative to; the rate of transfer to the non-active domain will produce coating with a narrow distribution. The model results shows that large beds widen the coating distribution as they are normally associated with a large non-active domain. Also, vigorous mixing of the bed was found to be instrumental in narrowing the coaling distribution and minimizing the effects caused by the presence of the non-active domain. The combination of bed size, rate of spraying and rates of exchange between domains can be used to obtain particles with a required coating distribution.