Thermodynamic models are used to calculate physical properties of petroleum fluids needed for process design and operation in petroleum related industries. Process simulators used for design and operational purposes are usually equipped with as many as a dozen of such thermodynamic models. The basic input parameters needed for use in these thermodynamic models are critical constants, acentric factor and molecular weight. In this paper, sensitivity of calculated properties from different characterization methods is demonstrated. Most commonly used characterization methods have been reviewed and based on evaluations with experimental data for various fluid mixtures recommendations are made for use of an appropriate model for specific hydrocarbon systems.