Methods of analysing neutron and X-ray specular reflection from interfacial systems are reviewed. Normally, the profile of the scattering-length density is determined in such experiments but here particular emphasis is given to the determination of the interfacial composition profile using partial structure factors and simultaneous fitting of sets of reflectivity profiles from a given structure, obtained either by isotopic substitution or by the use of neutrons and X-rays together. Aspects of the analysis of reflectivity data in terms of the resolution of the experiment, the phase problem and the possible ways of describing the structure of an interface are considered with reference to an unusually large set of independent data from isotopic species of a monolayer of hexadecyltrimethylammonium bromide adsorbed at the air/water interface. Data from another surfactant, the monododecyl ether of triethylene glycol, is used to assess the optimum choice of isotopic composition for combining a single set of neutron data with an X-ray reflectivity profile from an adsorbed layer at the air/water interface.