In the present work, a thermodynamic approach capable of describing the hydrogen behavior during its cooling and liquefaction is proposed both for the case of catalytic ortho to para conversion occurring inside dedicated reactors and for the case of continuous conversion inside heat exchangers where the catalyst is packed on the hydrogen side. The state-of-the-art Equation of State to describe the properties of normal-, para- and ortho-hydrogen is the Helmholtz free energy explicit equation. However, it can only describe pure components and not mixtures. The novelty of the proposed approach is that it is based on the widespread Peng Robinson Equation of State and that it allows to accurately describe the calorimetric and volumetric properties of the different forms of hydrogen and their mixtures. Furthermore, it can be easily implemented in the Aspen Plus® process simulator, resulting to be useful in view of design and optimization of the hydrogen liquefaction process.