The interfacial behavior of a short-chain poly(ethylene oxide) (PEO) lipopolymer, PEO (molecular weight 2000) grafted distearoylphosphatidylethanolamine (PEO2K-DSPE), and its mixtures with distearoylphosphatidylcholine (DSPC) have been studied using Langmuir film balance and surface tension methods. Surface pressure-area isotherms for pure PEO2K-DSPE and PEO2K-DSPE/DSPC mixtures (with 2, 5, 10, 15, or 20 mol % of PEO2K-DSPE) revealed two phase transitions. The simple additivity in the molecular area of lipid mixtures with increasing incorporation of PEO lipopolymer below the low-pressure transition indicates the existence of a two-dimensional thin layer of PEO and lipid. The low-pressure transition is interpreted as desorption of surface-adsorbed PEO molecules into the aqueous subphase. Correlation of film balance experiments with PEO lipopolymer and surface tension experiments with PEO aqueous solutions suggests dehydration of the PEO in the lipopolymer film by expulsion of PEG-associated water molecules as surface pressure increases. Theoretical models, based on scaling theories of polymers at interfaces, fit the observed isotherms well in the low-pressure regime below the first transition but poorly in the high-pressure regime above the first transition. The high-pressure transition is interpreted as ordering of lipid tails in lipopolymers, as driven by the enthalpic gain from lipid alignment and balanced by the entropic loss for lipid ordering and most importantly by dehydration of the PEO chains.