Multiple xylose and glucose degradation pathways were found previously depending on the initial protonation site on the sugar molecules and the water structure surrounding the sugar molecules.   Here the temperature and water density effects on the sugar degradation pathways were explored.  We found that both temperature and water density could change the reaction mechanisms.  Temperature affects hydrogen bonding interaction energy between the hydroxyl groups of the sugar molecule and the water molecule, and between the water molecules themselves.  Water density affects the average hydrogen bonded water cluster size, thus its proton affinity.   Both hydrogen bonding interaction energy and proton affinity were found to affect the sugar degradation pathways profoundly.