The work carried out in this thesis involves the investigation of some reactions of heterocyclic compounds by a wide range of spectroscopic and kinetic techniques. Chapter 1 describes several methods, including stopped-flow spectrophotometry, high pressure kinetics and MNDO calculations, which are used to analyse the mechanism of the reaction of imidazole with diazonium ions. Chapter 2 describes the use of the temperature-jump technique to follow the extremely fast protonation of pyrroles. Diazonium ions are further investigated in Chapter 3 which examines the structure of p-nitrobenzenediazonium tetrafluoroborate by X-ray crystallography. The nature of other benzenediazonium ions and some of their complexes is also investigated by solid state 15N nmr spectroscopy. Chapter 4 switches the emphasis to nmr spectroscopy. The reactive intermediates of reactions of benzil with nitrogen heterocycles are investigated using 13c nmr spectroscopy in a kinetic application. Chapters 5 and 6 continue the use of nmr spectroscopy in the structural assignment of bilirubin and the determination of the nature of bilirubin inclusion complexes. Chapter 6 also examines by spectrophotometry the reaction used in the clinical analysis of bilirubin and the effect on this reaction of binding agents such as albumin and α-cyclodextrin.