The structures of monolayers of Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate) adsorbed at the oil/water, air/water and hydrophobic solid/water interfaces have been determined using neutron specular reflection in conjunction with isotopic substitution. The areas per molecule were found to be 78 ± 2 9 2 and 80 ± 5 9 2 at the critical micelle concentration (CMC = 2.5 10 - 3 M), increasing to 132 ± 5 9 2 at 8.0 10 - 6 M and 213 ± 20 9 2 at 2.5 10 - 6 M at the air/water and hydrophobic solid/water interfaces respectively. The thicknesses of the AOT layer at the air/water and solid/water interfaces were found to be 18 ± 2 9 and 15 ± 2 9 with no significant change with concentration. At the oil/water interface the thickness is much larger than the fully extended molecule and is 26 ± 2 9 at an area per molecule of 68 ± 2 9 2 . This is probably caused by roughening of the oil/water interface by thermal fluctuations.