Isopiestic measurements have been made on the quaternary system (mannitol + sorbitol + D-glucose)(aq) and its two ternary subsystems (mannitol + D-glucose)(aq) and (sorbitol + D-glucose)(aq) at T = 298.15 K. The results are in excellent agreement with the semi-ideal hydration model and the partial-ideal solution model. Activity coefficients have been calculated for each solute in the mixed systems from the isopiestic results.