A technique of differential profilometry is reported which allows direct measurement of the deposits formed on a membrane. The technique is described and demonstrated to be capable of resolving deposits of one mum or less. Simultaneous measurements of the flux collected in a segmented manner along the operating membrane allow the calculation of the resistivity of the deposit formed. Microfiltration experiments at different velocities result in thinner deposits at higher velocities and higher fluxes. The deposits initiate downstream of the entrance, build somewhat erratically, then inexplicably diminish near the downstream end of the membrane. The method may lead to optical methods capable of continuous observations during operation.