The thermodynamic behavior of various types of plasticized ethyl cellulose with a degree of substitution (DS) in the range of 1.7 less than or equal to DS less than or equal to 2.5 is presented. Three structurally related polymer-plasticizer systems with different polarities were investigated, namely ethyl cellulose (DS 2.5)-glycerol tributyrate, ethyl cellulose (DS 2.1)-tributyl citrate, and ethyl cellulose (DS 1.7)-diethyl tartrate. The change in the specific volume of the materials was measured over a temperature range of 20-190 degrees C and a pressure range of 10-100 MPa, and the scaling parameters of the Simha-Somcynsky and the Flory-Orwoll-Vrij equations of state were determined and compared. Good agreement with experiment was obtained for both equations of state. The cohesive energy density (CED) and, in the case of the Simha-Somcynsky equation, the occupied volume fraction (y) of the amorphous mixtures were calculated at thermodynamic equilibrium, that is at temperatures above the relevant glass transition temperature (T-g). The T-g values of the binary mixtures were determined by the dynamic-mechanical method and corrected empirically to yield the low temperature limit of the thermodynamic equilibrium domain. The occupied volume fraction is strongly dependent on presssure and temperature and, above T-g, decreases with increasing amounts of the plasticizer component. As a result of the comparatively narrow pressure range considered in the evaluation of the volumetric data, the magnitude of y was found to be hardly comparable for the three systems investigated, although it accurately reflects the change in the specific Volume in each case. The CED generally appears to correlate with the hydrogen bond density of the mixtures and consequently increases with decreasing DS of the polymer component. The CED provides valuable information for the description of intermolecular forces between the segments in the modeled quasi-lattice and may be capable of approximating the size distribution of the unoccupied volume. Hence, both parameters are of outstanding importance in the study of factors that affect the diffusion coefficient of small apolar gas molecules in materials applied as film coatings in product forms of pharmaceutical agents and food additives. In this context, the results of the present contribution were used to discuss the oxygen permeability of plasticized ethyl cellulose in a subsequent paper.