Two plasticizers namely, glycerol and xylitol, based on their similar molecular diameter (similar to 6.3 angstrom) but different molecular weights (glycerol-92; xylitol-152) were selected were selected for studying the plasticization of starch biopolymer containing 70% amylopectin structure via glass transition measurements carried over a wide range of water activity. A standard calorimetry was used to determine the onset temperature of polymeric viscous flow. For both glycerol and xylitol, typical antiplasticization was evident at low plasticizer concentrations, whereas at higher concentration, there was significant reduction in glass transition temperature. Water activity isotherms showed that equilibrium moisture content of the starch biopolymer (no plasticizer) steadily increases up to 11%, however, for plasticized biopolymer, the moisture content was nearly double than that of biopolymer. We used a modified Gordon-Taylor model, using a new interaction parameter, to understand the competitive plasticization of glycerol and xylitol in presence of water, and determined 8 wt % water as a threshold amount of matrix water for strong three-way interactions: starch-plasticizer, plasticizer-plasticizer/water and starch-water. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 486-495, 2010