Equipment for on-line counting and sizing of particles has been used to sample coolant from the primary circuit of a water reactor (the Winfrith steam generating heavy water reactor). The particle size distribution is compared with a determination by electron microscopic examination of a filter sample and is shown to be in good agreement. The technique allows transients in coolant-borne particle concentrations to be sufficiently resolved for analysis in terms of postulated particle deposition and resuspension behavior. The deposition behavior is found to be describable by a first-order rate process with rate constants smaller than those that would be predicted from mass transfer considerations. It is concluded that deposition cannot be limited by mass transfer alone.