Poly-N-4-decyloxybenzoylethyleneimine, highly crystalline in bulk, forms a stable monolayer at the air-water interface which undergoes a reversible collapse into a probable bilayer structure at higher surface pressures, We achieved transfer both from the monolayer and "bilayer" region of the polymer film at the water surface onto hydrophilic and hydrophobized quartz supports. Based on X-ray investigations on the deposited Langmuir-Blodgett films, a possible mechanism for the reversible collapse is proposed.