The effects of dissolved carbon dioxide (CO2) molecules on the glass-transition temperature as well as the crystallization kinetics of poly(lactic acid) have been investigated with a novel device that combines ultrasonic and volumetric measurements. Ultrasonic parameters such as the sound velocity are very sensitive to crystallization and can be used to monitor the crystallization kinetics. The glass-transition temperature has been found to decrease nonlinearly as the CO2 concentration increases. The maximum in the crystallization rate has been found to increase with the addition of CO2. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1345-1355, 2009