The structure of the mixed nonionic surfactant monolayer of monododecyl triethylene glycol and monododecyl octaethylene glycol adsorbed at the air-water interface has been determined using specular neutron reflection. Using partial isotopic labeling (deuterium/hydrogen) of the alkyl and ethylene oxide chains of each type of molecule, the distribution and relative positions of those labeled fragments have been obtained. The frustration caused by the packing of the triethylene and octaethylene glycol headgroups results in a change of the surfactant structure compared to the pure monolayer of either surfactant. Compared to the pure monolayer the alkyl chain distributions of both surfactants are more extended, the triethylene glycol group is less hydrated, and the octaethylene glycol group is less extended and more hydrated.