High frequency backpulsing is a promising technique of flux enhancement that could contribute to the development of cross-flow micro-/ultrafiltration in water and wastewater treatment. A systematic study of the influence of the operational parameters was carried out with three suspensions, bentonite in tap water, biologically treated wastewater and activated sludge. The alumina membranes were tubular (0.02, 0:05 or 0.2 mum), with internal or external skin, the latter being not suitable. The technique was particularly efficient for bentonite; a minimal cross-flow velocity was required to reach a net flux independent of the cross-flow. The results are less good for the biological suspensions since the same fluxes could be reached by an increase of cross-flow velocity. However, the energy required by high frequency backpulsing is lower. The average reverse fluxes, measured by a tracer method, are surprisingly high and could hamper the development of the technique. At low Reynolds number (Re = 3500), the net flux increased with the reverse flux, then reached a plateau corresponding to the total penetration of the laminar layer against the membrane wall by the backwash water.