The calcium sulfate-water vapor system is of great scientific and technological importance due to its applications in several fields such as the construction 0.75 materials industry, geology, and planetary sciences. While much effort has been concentrated during the past decades on characterizing the crystallographic structure of the different calcium sulfate polymorphs, some questions concerning their thermodynamic aspects as phase equilibria and their capability to increase their overall water content continuously beyond structural water content seem to have been left aside. Nevertheless, the comprehension of these aspects is of the utmost importance if we want to understand this chemical system fully. The present two-part work investigates these phenomena experimentally and by a thermodynamic modeling approach. In this first part, we develop a rigorous experimental protocol by thermogravimetric analysis under controlled temperature and water vapor partial pressure. We use this protocol to obtain thermodynamic equilibrium values for the overall water content of calcium sulfate hydrates. To ensure that the equilibrium was reached, we verified that these values could be obtained by distinct thermodynamic paths. With the equilibrium data, we were able to propose an updated equilibrium curve between soluble anhydrite AIII-CaSO4 and CaSO4 center dot 0.5H(2)O and estimate the thermodynamic parameters Delta H-r degrees = (35.5 +/- 1.0) kJ center dot mol(-1) and Delta S-r degrees = (80.0 +/- 2.8) J center dot mol(-1)K(-1). After that, we were able to quantify the extent of water adsorption as a function of (T, P-H2O), and we observed that it could represent a significant part of the overall water content of calcium sulfates.