Immobilized Candida antarctica Lipase B (Novozyme-435) was studied for bulk polyesterifications of linear aliphatic hydroxyacids of variable chain length. The products formed were not fractionated by precipitation. The relative reactivity of the hydroxyacids was 16-hydroxyhexadecanoic acidapproximate to12-hydroxydodecanoic acidapproximate to10-hydroxydecanoic acid (DP(avg)congruent to120, M-w/M(v)less than or equal to1.5, 48 h, 90degreesC)>6-hydroxyhexanoic acid (DP(avg)similar or equal to80, M-w/M(n)less than or equal to1.5, 48 h, 90degreesC). Remarkable improvements in molecular-weight buildup resulted from leaving water in the reaction. By 4 h, without application of vacuum, the DPavg for 12- and 16-carbon hydroxyacids was about 90. In contrast, with identical substrates and water removal, the DPavg at 4 h was about 23. Large differences in the molecular-weight build up of 12-hydroxydodecanoic acid were observed for catalyst concentrations (%-by-wt relative to monomer) of 0.1, 0.5, 1, and 10. Nevertheless, by 24 h, with 1% catalyst containing 0.1% lipase, poly(12-hydroxydodecanoic acid) with M-n 17600 was formed. For 12-hydroxydodecanoic acid polymerization at 90degreesC, the catalyst activity decreased by 7, 18, and 25% at reaction times of 4, 24, and 48 h, respectively. Furthermore, the retention of catalyst activity was invariable as a function of the substrates used.