A fundamental equation of state is presented for the calculation of thermodynamic properties of chlorine. It is expressed in terms of the Helmholtz energy with the independent variables temperature and density. Based on the available experimental data from the literature, the equation is valid from the triple point temperature 172.17-440 K with a maximum pressure of 20 MPa. The quality of the equation is evaluated by comparisons with experimental measurements. Since the equation was developed in part for use in mixture models relevant for carbon capture and storage applications, special focus was also given to correct extrapolation behavior. K E Y W O R D S chlorine, equation of state, Helmholtz energy, thermodynamic properties 1 | INTRODUCTION Knowledge of thermodynamic properties is essential for many processes in chemical and process engineering as well as academic research. For industrial use, the properties have to be as accurate as possible and, additionally, easily accessible. In order to fulfill these requirements, equations of state are used. These equations are correlated to thermodynamic properties obtained with experiments carried out in laboratories. Therefore, the accuracy and range of validity of the equation of state is limited to the accessibility as well as the quality of the experimental data. The availability of thermodynamic properties is often quite limited for fluids that are dangerous to handle, for example, because they are toxic, flammable, or reactive with materials that are mounted in the apparatus. In this article, a new equation of state is presented for chlorine.Chlorine was discovered accidentally by Carl Wilhelm Scheele in 1774 by bleaching leaves and flowers. Its name was given by Humphry Davy in 1810 who realized that in fact chlorine is an element. In the following centuries, it became a very important substance in many different processes, for example, it is used in water purification, bleaching, medicines, paper production, and organic chemistry. At standard conditions, it is a greenish-yellow gas with a very strong smell. Because of its binding propensity, it is highly reactive with many substances. It is rarely found as an element in nature, but rather in combination with other substances, for example, sodium chloride. It is not flammable and only explosive