We have determined the structure of a monolayer of monododecyl octaethylene glycol (C(12)E(8)) adsorbed at the air/water interface at its critical micelle concentration and at temperatures of 298 and 323 K using neutron specular reflection in combination with isotopic labeling. There is little effect of temperature on the coverage of the pure material, though a significant variation is observed when the material was slightly contaminated with lower members of the series (C(12)E(m) With m < 8). However, subtle changes in the structure of the adsorbed layer do occur, the alkyl chain region becoming about 14% thicker at the higher temperature, whereas the thickness of the ethylene glycol chain region does not change. An analysis of these differences and the changes in the separation of the different fragments of the surfactant suggest that the layer is increasingly roughened at the higher temperature and that the ethylene glycol chain region is significantly dehydrated. The structure of C(12)E(8) is compared with other members of the C(12)E(m) series at approximately the same area per molecule of 55-65 Angstrom(2). The alkyl chain thickness is constant throughout the series, and the values of the structural parameters indicate a large average tilt of the surfactant molecules away from the surface normal and a significant incidence of gauche conformations in the alkyl chain.