In this study, we examined the role of environmental parameters and physical structure in the aging process of poly(lactic acid) (PLA). The role of heating history on the aging behavior of the material was also investigated. PLA samples with a D-content of 4.25% were exposed to a relative humidity of 80% at three different temperatures, 20, 40, and 50 degrees C (below the glass-transition temperature of the material), at various aging periods of 30, 60, 80, 100, and 130 days. Selected samples were subjected to two consecutive heating runs. The stability of PLA was monitored by a number of techniques, including size exclusion chromatography, differential scanning calorimetry, dynamic mechanical analysis, and tensile measurements. The initial thermal processing (150 degrees C) of the material resulted in an overall molecular weight reduction. A substantial lowering of properties was observed for PLA samples aged at 20 degrees C for 30 days. No further loss of properties was observed for samples aged up to 40 degrees C for several time intervals. A major portion (80-90%) of the induced changes in the tensile properties could be reversed after drying. At 50 degrees C and 100 days of aging, a sharp decrease in the overall properties was noticed. The results seem to confirm the earlier finding that PLA degradation driven by hydrolysis needs a higher temperature (> 50 degrees C) in combination with ample time to take place. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 472-481, 2011