Isopiestic measurements have been made for the systems, NaCl-NaBr-NH4Cl-mannitol((sat))-H2O, NaCl-NaBr-mannitol((sat))-H2O, NaCl-NH4Cl-mannitol((sat))-H2O, NaBr-NH4Cl-mannitol((sat))-H2O and KCl-CdCl2-mannitol((sat))-H2O at 298.15 K. The linear isopiestic relations presented previously have been derived in more detail using the semi-ideal hydration model. The results agree well with these linear relations for the first system and its three subsystems but not for the last one. These results, together with those published previously, indicate that for the saturated system, B-C-D-A(2(sat))-A(1), to obey the new relations, it is necessary that the unsaturated subsystem, B-C-D-A(1), conforms to the Zdanovskii's rule and the Stokes and Robinson relation.