In the early 1990s, enzymatic polymerization studies on an unactivated adipic acid (A) / butane-1,4-diol (B) system in anhydrous organic media suggested that in solvent-free conditions a step-growth mechanism operates involving the effective sequential addition of an 'AB' unit. Subsequent optimisation work led to development of an efficient solvent-free process using lipase B from Candida antarctica as catalyst at temperatures of around 60°C, producing polyesters with unique properties compared to their counterparts synthesised using conventional high temperature processes. The present work describes the synthesis and unambiguous characterisation by GPC of a series of key oligomers formed during the enzymatic process, enabling a clearer understanding of the polymer assembly route. Use of semi-quantitative methods to compare the reactivity of these oligomeric species along the proposed enzymatic reaction pathway further clarifies the polymerisation mechanism and explains the differences between the final products of conventional and enzymatic polymerisations. Comparison of polyesterifications performed in solvent-free and toluene-based media reveals a change in lipase specificity from a simple esterification to an esterification! transesterification mode. Addition of a variety of compounds to the enzymatic polyesterification process led to an enhancement of lipase activity and sometimes enabled recovery of lipase from the system with no loss in activity. Explanations for these effects are proffered with supporting experimental evidence. A variety of studies on the effect of changing the polarity of reaction medium have been performed, an apparent trend of decreasing lipase activity with increasing solvent polarity having been noted. The low temperatures employed in the enzymatic polymerisations enable access to a range of polyesters precluded by harsh conventional conditions. Studies on the polymerisation of novel polymers possessing unsaturation, labile groups such as epoxides and chirality are detailed, giving an indication of one of the potential advantages of an enzymatic system. In addition, studies on the more conventional uses of enzymes in organic synthesis are included, where an apparent switch in lipase stereoselectivity occurs, in line with the findings of other workers in the field.