We used molecular dynamics simulations to determine the sucrose hydrogen bond network (HBN) in amorphous sucrose with 0-50% w/w water. We find that the onset of a percolated sugar HBN network (between 33% and 40%) coincides with the experimental observation of the decoupling of viscosity and probe diffusion for these mixtures. The analysis of the free volume (FV) in these mixtures shows a non-monotonic behavior with water content which is consistent with experimental observations of hydrogen bond compaction and negative apparent partial volume of dilution, but in disagreement with the FV theory model of plasticization by water. (C) 2003 Published by Elsevier B.V.