The widespread occurrence of a large variety of long chain fatty acids has encouraged considerable investigation into the methods by which they are biosynthesised. The overall mechanisms of many of these pathways have been elucidated, but much remains to be discovered about the absolute stereochemistry of these reactions. This thesis describes work carried out to help define the stereochemistry of some of these reactions. To investigate the stereochemistry of two of the intermediate steps of chain elongation in fatty acid biosynthesis, the synthesis of αβ-unsaturated and β-hydroxy fatty acids specifically labelled with either tritium or deuterium was undertaken. The incubation of these labelled acids with the green algae Chlorella vulgaris allowed the absolute stereospecificity of the dehydration and hydrogenation steps of acyl chain elongation to be determined.