Three well-defined graft copolymers and a polynorbornene backbone with a poly(ethylene oxide) graft on each backbone repeat unit have been synthesized with fully hydrogenous and deuterated poly(ethylene oxide) grafts. The graft copolymers have a backbone degree of polymerization of 50 and the degrees of polymerization of the poly(ethylene oxide) grafts in the separate copolymers have values of 15, 25, and 50, Each of these copolymers has been spread at the air-water interface as Langmuir films with surface concentrations from 0.3 to 4.0 mg m(-2). The spatial extent of the regions occupied by the backbone and the grafts has been determined by neutron reflectometry. The polynorbornene backbone is confined to the air phase and contains an appreciable amount of air, the volume fraction of which increases as the surface concentration of graft copolymer increases. The poly(ethylene oxide) grafts are essentially totally immersed in the aqueous subphase, the distribution of ethylene oxide segments being described by a combination of a uniform layer with a parabolic tail. This description applies for all surface concentrations. The total thickness of the poly(ethylene oxide) layer increases with surface concentration in manner that is very similar to that predicted by single-chain mean-field theory for brushlike layers formed by block copolymers where there is a repulsive interaction between the tethered blocks and the anchor blocks. At the highest surface concentration the layer thickness is ca. 3-5 times the radius of gyration of the poly(ethylene oxide) grafts. There is no evidence for a first-order transition in the layer thickness at a critical surface concentration of the graft copolymers. Individual thicknesses of the uniform layer and parabolic regions show quite different behavior. The parabolic region thickness approaches an asymptotic value at high surface concentrations. The uniform layer thickness at first shows no change in layer thickness as surface concentration increases but then begins to increase abruptly at a surface concentration that increases as the degree of polymerization of the poly(ethylene oxide) grafts decreases.