Deuterium labelled linear diblock copolymers of polyethylene oxide and polymethyl methacrylate were spread at the air-water interface and neutron reflectometry was used to determine the spatial organization of the copolymer. The mole fraction composition of the block copolymer was ca 0.5, but with some variation from copolymer to copolymer arising from the separate syntheses needed for each labelled copolymer. The neutron reflectometry data for two surface concentrations of the block copolymer (0.6 and 1.2 mg m(-2)) were analysed using the kinematic approximation. In addition to providing the composition and thickness of regions which are rich in one component or the other, this method also gives the separation between each of these regions. For the lowest surface concentration used the two blocks were mixed together near the surface with a Gaussian distribution of segments, the characteristic dimension of which was circa 8 Angstrom. At the higher surface concentration of 1.2 mg m(-2), there is a clear separation between the two blocks with the polyethylene oxide exploring the aqueous subphase and part of the polymethyl methacrylate block being expelled into the air phase. However, there is no evidence for the formation of a stretched brush like layer of polyethylene oxide at the surface. Attempts to analyse for such a layer are discussed and the care needed in such analyses is pointed out. For both concentrations the near surface distribution of water takes on a tanh distribution.