A model describing the deposition rate of small particles on spherical collectors under turbulent flow conditions was derived. The derivation was based on several assumptions, the most important ones being that the small particles occupy an area equal to its projected area on the surface of the collectors and no detachment takes place. Three cases were considered: the small and the collector particles are both monodisperse in size (case I), the small particles are polydisperse while the collector particles are monodisperse (case II) and finally the small and the collector particles are both polydisperse in size (case III). The log normal distributions were used to represent the polydisperse sizes.