The application of new technique of specular neutron reflection to the study of the solution/air and solid/solution interfaces is reviewed. Examples are given of the determination of the structures of adsorbed layers of tetramethylammonium dodecyl sulphate (TMDS) and decyl trimethylammonium bromide (DTAB) at the air/liquid interface. In both of these systems the layer consists of an outer alkyl chain region, which is not close-packed and which contains no water, and a head group region containing the head groups, counterions, water and a small fraction of alkyl chains. The non-ionic surfactant hexaoxyethylene glycol monododecyl ether (C12E6) is shown to form a bilayer at the quartz/aqueous solution interface at concentrations close to the c.m.c. However, this bilayer is found to be defective and might be better described as a layer of flattened micelles. The structure persists down to coverages where less than half the geometric surface is covered with bilayer.
