We have determined the structure of a monolayer of dodecyltrimethylammonium bromide (C(12)TAB) adsorbed at the air/water interface at three surface concentrations corresponding to areas per molecule of 44, 48, and 60 Angstrom(2), using neutron reflection. We have used isotopic labeling to distinguish the hydrocarbon chain, head group, and solvent distributions. We have also labeled the two halves of the hydrocarbon chain to obtain information about the mean conformation of the chain. At the highest surface concentration we find that the inner half of the chain (next to the head group) is oriented closer to the surface normal than the outer half of the chain indicating that chain defects contribute to the chain structure. We also find that the surfactant layer is substantially rougher than that predicted from the simple capillary wave model of a pure liquid. We have compared the structure of the different C(n)TABs with chain lengths varying from C-12 to C-18 at an area per molecule of about 44 Angstrom(2) and from C-10 to C-16 at an area per molecule close to 60 Angstrom(2). The most remarkable observation about the lower area measurements is that the thickness of the layer is the same for the four chain lengths even though the extended length of the chain increases by 50%. When account is taken of surface roughness, the intrinsic thickness of the chain region of the monolayer is also constant and for C-18 is less than half the extended length. Comparison of the thickness of the layer and the separation of the chain and head distributions indicates that the mean orientation of the outer half of the chain is further from the surface normal than is the inner half of the chain, and this difference increases with chain length. At the higher area per molecule the C(10)TAB forms the thickest layer and this is associated with the chains being closer to the surface normal than for the other C(n)TABs.