We have determined the structure of a monolayer of dodecaethylene glycol monododecyl ether (C12E12) adsorbed at the air-water interface using specular neutron reflection in combination with deuterium labeling. The structure of the adsorbed monolayer was measured at its critical micellar concentration (cmc, 1.25 x 10(-4) M), 0.3cmc, 0.13cmc, and 0.033cmc. The reflection data were analyzed using the optical matrix method and the kinematic approximation, both of which gave structures identical within tile limitations of the different models used. Fn the optical matric; method the monolayer was divided into two layers, the first layer consisting of the alkyl chain only, and the second layer consisting of the ethoxylated head, a fraction of the alkyl chain, and water. Significant intermixing of alkyl chain and ethoxylated headgroup was needed to account for the observed reflectivities. At the cmc about 35% of the alkyl chain seems to be incorporated into The headgroup region, and this increases to about 50% at the lowest concentration. The corresponding variation in the thickness of the alkyl chain layer was from 9 to 6 +/- 2 Angstrom and that of the head region from 22 to 20 +/- 2 Angstrom. The distances between the centers of the alkyl chain and solvent distributions were determined by the more direct kinematic method and the values were found to be 13, 11, 10, and 7.5 +/- 2 Angstrom for the four concentrations studied. The total thicknesses of the dodecyl chain and the ethoxylate groups were extracted from a simultaneous analysis of three reflectivity profiles from different isotopic compositions at a given surfactant concentration. The values of the two thicknesses were found to be 15.0, 13.0, 12.5, and 12.0 +/- 2 Angstrom for the dodecyl chain and 21, 18, 17, and 17 +/- 4 Angstrom for the ethoxylate group at the four concentrations. Comparison of these values with previous work on C12En with shorter ethylene glycol chains shows that the distributions of the dodecyl chain are similar to those with smaller n, but the extent of mixing between the allyl chain and the ethoxylate group increases with n.