Thiourea, 1-substituted thioureas and 1,3-disubstituted thioureas react under acidic or basic catalysis with 1,2-diketones to give a wide variety of products. The nature of the product or products depends on the conditions used, the degree of substitution of the thiourea and on the nature of the aromatic or aliphatic groups attached to the dicarbonyl unit of the diketone. The mechanisms of formation of the various products have been investigated by a variety of techniques, including the determination of rate equations by ultraviolet spectroscopy, the influence of side group substituents on the rate and course of the reactions and the synthesis of putative Intermediates on the reaction pathway. The single most informative technique used was, without a doubt, the detection of low concentrations of transient intermediates and side products by the application of carbon-13 nmr spectroscopy to the reaction as it progressed using carbon-13 labelled di ketones to enhance the detection of low level intermediates. It has been shown convincingly that some organic reactions proceed via a small number of irreversible processes amongst a very complex set of more rapid equilibrium processes.