For the first time, the formation and growth of a molecular superstructure within a lipopolymeric monolayer on the water surface is directly observed by small-angle grazing incidence diffraction. Nanoscopic stripes of ordered alkyl chains and solubilized polymer alternate.(1) At the onset of the superstructure formation, the smallest stripe periodicity is measured. Then, the periodicity relaxes (up to 170 Angstrom), even if the monolayer is further compressed. This threshold behavior is attributed to a very high local pressure necessary for stripe alignment. On solid substrates, the superstructure periodicity is decreased down to 120 Angstrom because of dehydration of the polymer moieties.