A polystyrene - poly(ethylene oxide) containing diblock copolymer, when spread at the air/water interface, spontaneously forms surface aggregates. This surface aggregation is shown to be neither compression-induced, associated with micellization in the spreading solvent, nor induced by the Langmuir-Blodgett film transfer process. We have previously found that such two-dimensional surface aggregation occurs for diblock copolymers with a polystyrene block and a hydrophilic block of quaternized poly(vinylpyridine), poly(tert-butyl acrylate), poly(n-butyl acrylate), or poly(dimethylsiloxane). The phenomenon has also been observed in films of polystyrene-poly(methyl methacrylate) by Rice and co-workers.＇ Indeed, whenever an appropriate imaging technique has been used, phase-separated domains with 30-100 nm length scales have been observed, when amphiphilic diblock copolymers are spread at the air/water interface and transferred to solid substrates at appreciable surface pressures. We therefore believe that the formation of surface aggregates (often well defined) is a general phenomenon for hydrophobic-hydrophilic diblock copolymers. The implications of this phenomenon for the study of diblock copolymers at the air/water interface are discussed, particularly in relation to studies using techniques which report properties averaged over the lateral dimensions of the film in question, such as specular neutron reflectivity, specular X-ray reflectivity, and Brewster angle microscopy.