Neutron reflection has been used to determine the position of the p-tosylate ion in monolayers of the hexadecyltrimethylammonium cation (C(16)TA(+)) at the air-water interface. The monolayer was in equilibrium with a solution of composition 1 mM C(16)TAB and 3 mM sodium tosylate. The area per molecule of the surfactant was found to be 0.45 and 0.52 nm(2) respectively in the absence and presence of tosylate and 0.47 nm(2) in the presence of 3 mM NaBr. The ratio of tosylate to C(16)TA(+) in the layer was found to be 0.8. Five isotopic species of the C(16)TA(+) were labeled with deuterium in blocks of four atoms (C4D8, C4D9, or headgroup (CD3)(3)N), and the separation between the center of each of these blocks and the deuterated tosylate ion was determined. The values show that the average position of the tosylate ion in the layer is at exactly the same level as the third block of four carbons in the C(16)TA+ (the first block is defined as that containing the terminal methyl group); i.e., the tosylate ion is mainly in the hydrophobic region of the monolayer. This conclusion is further confirmed from the separation of the average tosylate ion and the midpoint of the water distribution, which was found to be 0.7 +/- 0.3 nm. Taken in conjunction with sum-frequency spectroscopy measurements on the same system, which give the orientation of the tosylate ion in the interface, and the effect of added sodium tosylate on the surface tension of C(16)TAB, the tosylate is shown to be acting much more like a cosurfactant than like a normal counterion.