Abstract. A simple method is proposed to partition a mixture of two populations in suspended particle size data. The method, termed here "the differentiation method" is based on the function of the lognormal distribution. Suspended material in marine or estuarine situations often consists of difficult-to-interpret complex populations. The treatment of particle size data by the method described enables the confirmation of the lognormal law and also the demonstration of the occurrence of a combination of a number of populations which may not be distinguished by the classical Gaussian transformation or automatic methods. A simple combination of graphical and numerical techniques permits the decomposition and the easy determination of the various statistical parameters (median diameter, mean diameter, etc...). The method is applied to interpret observed size distributions of suspended particulate matter in the Seine estuary. The method enables the determination of the relative sizes of the constituent sub-populations that comprise the total suspended matter. In the example used to illustrate the method, particles are shown to be resuspended as a function of different hydrodynamic parameter.