The techniques of ellipsometry, X-ray reflection and neutron reflection will be used to investigate the interfacial structure associated with the critical adsorption of a binary liquid mixture at a consolute point. Each of the techniques (when applicable) can either provide an extra facet to or else provide complementary evidence for the structure determination. It is intended to investigate two classes of mixture: (alkane + perfluoroalkane) and (water and/or heavy water + polyethyleneglycol ether surfactant). The former class of mixture is favourable for the determination of the mathematical form of the interfacial profile, whereas the (water + surfactant) mixtures exhibit some unusual depletion-layer behaviour prior to the critical adsorption profile and are therefore interesting from the viewpoint of intermolecular forces. For the investigation of this class of mixture by neutron reflectivity it is necessary to substitute water with heavy water. Initial ellipsometric results have shown that when this substitution is made no depletion layer forms. By using mixtures of water and heavy water with the surfactant the point of crossover and the nature of the interactions which govern the depletion layer formation can be probed.