Fish is a highly perishable foodstuff best preserved by low temperature storage but even under these conditions deterioration still occurs and off-flavours are produced. This thesis describes an investigation into the nature, origin and mechanism of development of the components responsible for this off-flavour in cod (Gadus morhua). An understanding of these matters may make it possible to prevent the development of this flavour or at least minimise the effect. In Chapter I the current theories of the perception and mechanism of taste and alfaction are described, as are the complexities of the chemical nature of flavour in food and especially fish. The causes of off-flavour development in fish are discussed and the background to the cold storage phenomenon introduced. The characterisation of the compounds responsible for the off-flavour is described in Chapter II. By using dinitrophenylhydrasone derivatisation it was found that carbonyls are largely responsible but these derivatives could not be used for a complete analysis. The technique that was finally used to fractionate the mixture was that of gas liquid chromatography supported by organoleptic assessment. In this chapter, the experimental procedures adopted in the investigation are described. In Chapter III the major component responsible for the cold storage flavour is shown to be present at extremely low concentrations in the fish and is identified as hept-cis-4-enal. Details of other volatile components present in cod flesh are also presented including two other aldehydes, hept-trans-2-enal and hepta-trans-2-cis-4-dienal which possess some of the off-flavour character. Chapter IV presents the evidence for the production of those compounds by an autoxidative pathway and gives information on their rate of production with temperature. Taste panel assessments carried out during the latter work showed that there was a positive correlation between increase in hept-cis-4-enal concentration and cold storage flavour. A more detailed study indicated that hept-cis-4-enal is produced by to 3 polyenoic fatty acid oxidation but it was not possible to resolve whether a specific fraction or the total lipid was involved in this production. The results show that in white fish the rate of oxidation in cold storage is slow despite the unsaturated nature of the lipids and an explanation for this is advanced in Chapter V where the general mechanism of autoxidation is discussed. Some consideration is also given to the method of formation of hept-cis-4-enal from the w 3 polyenoic acids. A number of possible routes have been identified but the most probable pathway is via one of the major peroxide Intermediates in the autoxidation.