The mechanism of crossflow membrane filtration of an ionic double chain surfactant dispersion is evaluated by studying the deposition of the surfactant on the membrane and within its pores, together with the transient behaviour of the permeate flux and permeate concentration. It is shown that the surfactant deposition results in the formation of a secondary membrane supported by the primary membrane. To a large extent, the secondary membrane controls the permeate flux and rejection of the surfactant. The secondary membrane is formed on the surface of the membrane and penetrates into the pores forming a region of high surfactant concentration which is an order of magnitude higher than the so-called (pseudo) gel concentration. The structure of the secondary membrane, evaluated from cold field emission scanning electron microscopy, differs from that expected on the basis of phase behaviour of the surfactant at the prevailing concentration, temperature and pressure.