The biodegradability of lactic acid based polymers was studied under controlled composting conditions (CEN prEN 14046), and the quality of the compost was evaluated. Poly(lactic acids), poly(ester-urethanes), and poly(ester-amide) were synthesized and the effects of different structure units were investigated. The ecotoxicological impact of compost samples was evaluated by biotests, i.e., by the Flash test, measuring the inhibition of light production of Vibriofischeri, and by plant growth tests with cress. radish, and barley. All the polymers biodegraded to over 90% of the positive control in 6 months, which is the limit set by the CEN standard. Toxicity was detected in poly (ester-urethane) samples where chain linking of lactic acid oligomers had been carried out with 1,6-hexamethylene diisocyanate (HMDI). Both the Flash test and the plant growth tests indicated equal response to initial HMDI concentration in the polymer. All other polymers, including poly (ester-urethane) chain linked with 1,4-butane diisocyanate, showed no toxicological effect.