Neutron reflection has been used to investigate the structure of adsorbed layers of two triblock copolymers with approximate formulas E 23 P 52 E 23 and E 9 P 22 E 9 , where E is ethylene oxide and P is propylene oxide. Measurements were made at the air/water interface at the critical micelle concentration and at two much lower concentrations, and at temperatures of 25 °C and 35 °C. Isotopic labeling was used to improve the resolution of the experiment. In general, the adsorbed layer can be described in terms of a minimum of four uniform sublayers. The outermost layer is always a water-free layer containing only propylene oxide (PO) residues. The ethylene oxide (EO) residues form a tail below this layer, which extends into the solution over a distance slightly shorter than the fully extended length. Depending upon the conditions, some PO is also mixed in with this tail region. For example, at the lower surface concentrations, the segment density of the (PO) decays rapidly and there is a relatively small proportion of PO immersed in the water but, at the highest concentrations, there is a much larger amount of PO in the aqueous/EO region, and this extends some way into the solution. At the higher temperature, there is little PO in the aqueous region of the layer, but there is significant mixing of PO with EO in a sublayer just below the main PO layer out of the water. The smaller molecular weight polymer gives a much more disordered structure with greater mixing of all three components. The disorder in these adsorbed layers is higher than that previously suggested for the micellar structure. Comparison of the layer structure at the air/water interface with that in micelles indicates either that there is a significant difference in the extent of mixing of the PO and EO between the two situations or that the published small-angle scattering data can be reinterpreted in terms of a micellar structure where the PO/EO mixing is more in accord with that observed at the air/water interface.