Lipopolymers are lipids with a polymer chain covalently attached to the lipid. We systematically studied the monolayer behavior of a series of lipopolymers by use of infrared reflection-absorption spectroscopy. (IRRAS) and classical Langmuir film balance techniques. The lipopolymers differed in chemical composition both in the lipid chains and in the polymers. However, the lipid chains contained 18 carbon atoms in all cases. We find the lipopolymers to show a very complex phase behavior at the air-water interface, depending on such diverse parameters as polymer hydrophilicity, length of polymer; and saturation of the lipid chains. We were able to measure up to two different plateau regimes in the monolayer isotherms. One is observed for lipopolymers where the polymer part is only slightly water-soluble and is correlated with the desorption of the: polymer from the water surface. This desorption transition can be termed a pancake-mushroom transition. The origin of the second plateau regime is more uncertain. Baekmark et al. (Langmuir 1995, 11, 3975) interpreted this plateau as a transition within the polymer (mushroom-to-polymer brush conformation), but in two recent publications, by Goncalves da Silva et al. (Langmuir 1996, 12, 6547) and Baekmark et al. (Langmuir 1997, 13, 5521) this interpretation has been questioned. In the present work we show that this second transition is native to lipopolymers in the sense that it can only be observed when a lipid and polymer are present in the same molecule. On the same basis of the film balance and the IRRAS data, we are able to show that the lipid alkyl chains must be involved in the molecular processes constituting the "native" transition. To account for the origin of the "native" transition, we suggest that a process of alkyl chain condensation coupled to a strong reduction in the number of gauche isomers within the alkyl chains supply the driving force for the "native" transition.