Specular neutron reflection combined with deuterium labeling has been used to study the structure and composition of a mixed layer of oil and surfactant at the air/water interface, formed by spreading ethyl hexadecanoate from small lenses placed on the surface of hexaethylene glycol monododecyl ether (C12E6) The area per oil molecule was found to be 128 +/- 10 Angstrom(2) in the mixed monolayer and that of C12E6 to be 54 +/- 3 Angstrom(2), identical to the value obtained in the absence of the oil. The thicknesses for the dodecyl chain layer and ethoxylate layer were 18.5 +/- 2 Angstrom and 20 +/- 2 Angstrom, assuming Gaussian distributions, as compared with the respective:values of 16 +/- 2 Angstrom and 16.5 +/- 2 Angstrom without the oil, suggesting that oil mixing has broadened the distributions of the two fragments. The extent of penetration of the ester oil into the surfactant layer was described by the relative distances between the centers of the distributions. The use of alkyl chain deuterated and ethoxylate headgroup deuterated surfactants allowed separate determinations of the relative distances between the centers of the distributions of the dodecyl chains and of the oil (delta(co)) and between the ethoxylate headgroups and the oil (delta(eo)). The value of delta(co) was found to he 0.5 +/- 1 Angstrom and that of delta(eo) to be 10.5 +/- 1 Angstrom, showing that while the center of the ethoxylate headgroups was further into aqueous solution the ester oil layer was almost, entirely overlapped with the distribution of,the dodecyl chain layer. On the basis of comparisons made,with the results from other systems involving the spreading of dodecane and dodecanol onto the layers of ionic surfactants and nonionic alkyl ethoxylates, we attribute the deep insertion of the ester oil into the C12E6 layer to the combined effect of the affinity of the ester groups toward water and the affinity of the oil toward the ethoxylate groups.