Recent work has revealed that the assumption regarding the behavior of gases (perfect, ideal, real) and, consequently, the way their properties are evaluated may alter critically the picture obtained about the performance of gas turbine systems. This fact prompted an investigation of how the aforementioned assumption may affect the optimal design point of gas turbine systems. The present study is restricted to a comparison between the ideal and perfect gas assumption. Three systems have been selected for study and three optimization problems have been formulated and solved for each system: two thermodynamic and one thermoeconomic. The results demonstrate that the method used for the evaluation of properties of gases has a very significant effect on the optimal point of each system.The article is published with the permission of theorganizers of ECOS 2005, where it was first presented