The work in this Article deals with the estimation of thermodynamic data for the major species found during lubricant degradation. The organic group R on which the species were based was the pentaerythritol molecule. A computer code was developed that utilizes zero-order and bond additivity methods for the estimation of thermodynamic properties. The validation of the code was carried out against thermodynamic data for species found in published databases and showed that the computed values for heat capacity, enthalpy, and entropy were in close agreement with the reference values. Having gained confidence in the reliability of the developed code, the thermodynamic properties of species involved in aviation gas turbine lubricants were estimated and are presented in this Article. This work is the first part toward a study for the development of a lubricant degradation model in which the changes in the thermodynamic properties, and therefore heat transfer, due to chemical reactions are accounted for.