The process of deep-bed filtration has been studied using a Coulter Counter to analyse the size distribution of the suspension at various depths in the filter and at various times. The results show that an understanding of deep-bed filtration based merely on the mean size of a suspension is inadequate, and that the size distribution must be taken into account. A mathematical model to predict the effect of size distribution on removal efficiency has been developed, and a qualitative description of the model is given in the paper. A comparison between the experimental and predicted results is presented. The model predicts that in certain circumstances the efficiency of filtration may be improved by ‘dosing’ the feed with larger particles.