Tritium has been used as an isotopic tracer to study the mechanisms of several reactions in which a variety of catalysts play a prominent role. The first chapter of this thesis deals with the mechanism of labelling by homogeneous metal catalysts with particular regard to the tetrachloroplatinate(II) ion and other Group VIII transition metal chloro complexes. A disagreement in the literature concerning the mechanism is clarified. The catalytic properties of some Lewis acid halides are studied in Chapter 2. These are particularly effective for hydrogen isotope exchange in aromatic compounds. The use of tritium gas in the production of tritiated compounds is reviewed in Chapter 3. Also described here is the design and operation of a manifold to handle multicurie quantities of tritium gas. Chapter 4 is concerned with non-specificity of labelling in tritiated folic acid and methotrexate. Use of tritium nmr spectroscopy has resolved some anomalous chemical degradation results. The final chapter contains a discussion of the mechanism whereby upon reduction of a terminal double bond with tritium gas and a heterogeneous metal catalyst a substantial proportion of label becomes attached to the resulting terminal methyl group. Analysis of multiple labelling patterns in [3H]dihydroalprenoiol by tritium nmr shows that alkene exchange precedes hydrogenation, resulting in unsymmetrical labelling.