Aqueous solutions of trehalose, sucrose, fructose, and glucose at concentrations from 0.2 to 70 wt % (mole fractions from 0.01% to 18.9%) have been investigated using modulated differential scanning calorimetry (MDSC). The apparent (nonequilibrium) heat of melting (H-m(ne)), heat of freezing (H-f(ne)), and depression of the melting and freezing temperatures have been measured as a function of sugar concentration. Under the fixed experimental protocol and at any fixed concentration, the trehalose solution shows the lowest H-m(ne) and H-f(ne) among the four sugars. This may be due to an increased interaction between this sugar and water molecules. Trehalose also has the highest glass transition temperature among the four sugars. These two facts may be linked with the role of trehalose in cryopreservation. We observed that, for some solutions, H-m(ne) increases with the lowering of cooling temperature and the total time the sample remains at temperatures below the equilibrium freezing point. This increase is likely to be associated with structural changes occurring in the ice.