The novel linear concentration relations and the simple predictive equations for the thermodynamic properties (chemical potential, activities of all B-J (J = 1, 2,..., j), Gibbs free energy, thermal properties, and volumetric properties) have been proposed for the multicomponent system B-1-B-2-...-B-j-C-1-C-2-...-C-k-A(1)-A(2)...-A(i) in terms of the concentration and the properties of its subsystems B-j-C-1-C-2-...-C-k-A(1)-A(2)-...-A(1) -A(2)-...-A(i) (J = 1, 2,..., j) of equal activities of all C-1, C-2, ..., C-k and A(1), A(2),..., A(i). At has been shown that the process of mixing the nonideal systems B-j-C-1-C-2-...-C-k-A(1)-A(2)-...-A(i) (J = 1, 2, ..., j) of equal activities of all C-1, C-2, ..., C-k and A(1), A(2), ..., A(i) shows ideal mixing behavior and that the newly established equations are the same in form as those for mixing the ideal systems B-J-C-1-C-2-...-C-k-A(1)-A(2)-...A(i) (J = 1, 2,..., j) of equal mole fractions of all C-1, C-2, ..., C-k and A(1)-A(2)-...-A(i). The thermodynamic basis for the ideal mixing behavior has been presented. It has been shown that, while Raoult's law can describe the ideal behavior of mixtures based on similar components, the present equations describe a new type of "ideal" behavior of nonideal mixtures B-1-B-2-...-B-j-C-1-C-2-...-C-k-A(1)-A(2)-...-A(i) at constant activities of all C-1, C-2, ..., C-k and A,, A2,..., Ai. The linear concentration relations have been tested by comparing with the isopiestic measurements for the saturated systems glucose-sorbitol-sucrose((sat))-water, glucose-mannitol-sucrose((sat))(-) water, sorbitol-glucose-mannitol((sat))-water, sorbitol-sucrose-mannitol((sat))-water, mannitol-sorbitol-sucrose((sat)) -water, mannitol-sucrose-urea((sat))-water, and sucrose-urea-mannitol((sat))-water at 298.15 K. As expected, the former six systems conform to the new relations very well, but large deviation is observed in the last system.