The provision of storage at overflows is a commonly used technique for the reduction of pollution from combined sewer systems. Field data were gathered at three combined sewer overflow sites during dry weather and a wide range of high flow events. The overflows incorporated off-line storage which at two sites took the form of rectangular partitioned tanks and at the third was a twin hydrodynamic separator installation. Conventional flow measurement and small bore sampling equipment was employed together with a prototype Gross Solids Sampler (GSS) manufactured by the UK WRc, and visible solids interception devices developed by the author and termed Trash Traps. A relationship was developed for the variation of visible solids during dry weather flow at the inlet to one site based on the GSS results. Good correlation was found with suspended solids concentrations allowing the relationship to have wider applicability. Retention of particulate matter during high flow events was found to be more dependent on volumetric considerations than on the treatment provided by the storage. It was found that for all sites studied the measure of pollutant separation at each installation, the treatment factor, did not vary significantly from unity. The Trash Traps provided a method of distinguishing between the performance of the overflows utilising the visible solids intercepted and the degree of blinding of the Traps. It was concluded from the GSS results that the gross solids arriving at the overflow sites had the same movement characteristics as the type C sediment which is usually found in sewer inverts. A chart which provides a basis for a differentiation between combined sewer categories was prepared. This was developed from the rate of gross solids movement and on the average flow during high flow events. The nomograph showed a clear distinction between a collector and a trunk sewer site and included antecedent dry period as a significant component.