The study compares the effects of aprotic (i.e., gamma-valerolactone, acetone, and 1,4-dioxane) and protic (i.e., methanol and ethanol) solvents on the primary decomposition reactions of glucose and fructose, which are key biomass-derived platform molecules, in hot-compressed solvent/water mixtures (10 vol %) at various temperatures. Aprotic solvents enhance dehydration reactions and suppress isomerization reactions during glucose and fructose decomposition. Particularly, 5-hydroxymethyl-furfural (5-HMF) that is a secondary product for both glucose and fructose decomposition in protic solvent/water mixtures becomes a primary product for their decomposition in aprotic solvent/water mixtures, contributing to similar to 25-27% and similar to 19 22% of the primary decomposition reactions for glucose and fructose, respectively. In contrast, the protic solvent has little effect on the primary decomposition of sugars in hot-compressed solvent/water mixtures. The addition of the aprotic solvent reduces the activation energy of sugar decomposition in the solvent/water mixture, but the reaction rate constant in the aprotic solvent/water mixture is still lower than that in the protic solvent/water mixture.