The method described previously [2,3 J for obtaining the relaxation times, normal coordinates and normal modes of relaxation of a chemical system is extended to the case in which heat transfer and volume changes occur at rates too slow to maintain isothermal and isobaric conditions. The four types of chemical relaxation treated previously (isothermal and adiabatic relaxation at constant pressure and at constant volume) form limiting cases of this problem whose solution thus provides a generalization and unification of the concept of orthonormal chemical reactions.In previous papers of this series [1, 2, 3J we confined our attention to chemical relaxation under four simple thermodynamic conditions (isothermal and adiabatic relaxation at constant pressure and at constant volume). These problems form limiting cases of the somewhat; more general relaxation problem treated here in which heat transfer and volume changes occur, though at rates too slow to maintain isothermal and isobaric conditions. We treat this problem by an extension of our previous method in which we look upon the entropy and volume of our system as if they were two additional chemical substances and assume that the entropy and volume changes which occur can be ascribed to two additional elementary "chemical" reactions.As in part 3, we consider a closed system which, at time t, has a uniform terrperature T and pressure P and contains Ni molecules of the ith reacting substance (i=1,2, ... ,r) and N s molecules of the solvent s. We define the molal ratio mi of the ith substance by (i=1,2, ... ,r) ( 1) and denote the corresponding chemical potential by ].Ii. In the present paper we suppose, however, that our system is in thermal contact with a heat bath at a constant terrperature~and that it is fitted with a piston working against a constant pressure pO • As before, we choose a suitable arbitrary reference equilibrium state corresponding to the terrperature~and pressure pO and to molal ratios m~, m~, ••• ,m'j. and denote the small deviations of our system fran this reference state