Preliminary enzymatic polymerization studies in the simple stoichiometric adipic acid/butane-1,4-diol system using lipase B from Candida antarctica, immobilized as Novozym 435(R), suggest that in solvent-free conditions a step-growth mechanism operates involving the sequential addition of an AB synthon by esterification mode only. Conversely, in toluene as solvent there is a change to the more facile transesterification mode in line with the conventional polyesterification procedure, pointing to a change in specificity of the lipase. Evidence is drawn from qualitative studies using a series of synthetic intermediates, enabling authentication of product mixtures together with an indication of the comparative reactivity of species along the proposed reaction pathway.