Many industrial applications deal with chemical reactive systems. In some cases, new species are formed in chemical reactions that are not stable as pure substances. These components are present only in the reaction mixtures, and normally, their physical properties are unknown. To describe the phase behavior of these mixtures, the chemical reactions, as well as the physical interactions, have to be taken into consideration. Thermodynamic models, such as G(E) models, take into account the interactions between these so-called true components. Other physical properties, such as density, viscosity, etc., are often described with empirical correlations. Quite often, these correlations are mixing rules using the physical properties of the pure apparent components weighted by their apparent concentrations. Herein, a general method using reaction invariants for the estimation of the concentrations of the apparent components, based on the true-component concentration for a single phase, is presented. The limits of the method in applications to calculations of phase equilibria are discussed.