The transient permeate flux and rejection behaviour of a cationic surfactant dispersion during crossflow microfiltration is studied using membranes with different degrees of hydrophobicity which is characterised by the solubility parameter. It is found that the transient permeate flux behaviour is strongly affected by the membrane hydrophobicity if the feed concentration is low. The mechanism of crossflow membrane filtration is evaluated by studying, in addition to transient permeate flux and rejection, the deposition of the surfactant on the membrane and within its pores. 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 gel concentration.