Plasma polymer thin coatings can be used in a variety of applications that require thin ultrasmooth, defect-free homogeneous films. In this study both film formation and crack development are described and related to the structure of the plasma polymer. The growth of a plasma polymer film reproduces the topography of the porous (polysulfone) PSf support and the time needed for complete coverage depends on the rate of deposition which is a function of the plasma power and monomer flow rate. The plasma polymerized hexamethyldisiloxane, PPHMDSO, film on a PSf porous support is under compressive internal stress causing the membrane to bend convexly with respect to the film. Despite these stresses, the composite membrane (film/support) did not crack when freely immersed in water and alcohols. Cracks developed in the mechanically constrained PPHMDSO composite membrane on exposure to alcohols. Films with more organic character showed high adhesion among the plasma polymer nodules and between the plasma polymer and the PSf substrate, resulting in substrate tearing and cracking straight through the plasma polymer layer.