The interfacial properties of amphiphilic linear diblock copolymers based on poly(ethylene oxide) and poly(e-caprolactone) (PEO-b-PCL) were studied at the air-water (A/W) interface by surface pressure measurements (isotherms and hysteresis expts.). The resulting Langmuir monolayers were transferred onto mica substrates and the Langmuir-Blodgett (LB) film morphologies were investigated by at. force microscopy (AFM). All block copolymers had the same PEO segment (Mn = 2670 g/mol) and different PCL chain lengths (Mn = 1270; 2110; 3110 and 4010 g/mol). Isothermal characterization of the block copolymer samples indicated the presence of three distinct phase transitions around 6.5, 10.5, and 13.5 mN/m. The phase transitions at 6.5 and 13.5 mN/m correspond to the dissoln. of the PEO segments in the water subphase and crystn. of the PCL blocks above the interface similarly as for the corresponding homopolymers, resp. The phase transition at 10.5 mN/m was not obsd. for the homopolymers alone or for their blends and arises from a brush formation of the PEO segments anchored underneath the adsorbed hydrophobic PCL segments. AFM anal. confirmed the presence of PCL crystals in the LB films with unusual hairlike/needlelike architectures significantly different from those obtained for PCL homopolymers.