For active pharmaceutical ingredients (API) in the solid state, the occurrence of hydration/dehydration phase transitions may have implications upon the efficiency of the pharmaceutical product, once they are often accompanied with changes of physicochemical properties. Thus, it is important for pharmaceutical research and development to investigate these mechanisms based on the crystal structures of the API. In this work, beyond the structural characterization of a new salt of the antidepressant drug Paroxetine, an HBr hemihydrate – (PRX+Br–)0.5H2O, by Single Crystal X-ray Diffraction (SXRD), Thermal analysis (DSC/TGA), and Hot Stage Microscopy (HSM), we were able to perform a complete investigation toward the reversible hydration/dehydration solid phase transitions occurring for this salt. In addition, solubility measurements for the anhydrous and hemihydrate solid forms of the new salt are compared with those of its isostructural hydrochloride hemihydrate salt, (PRX+Cl–)0.5H2O, for which hydration/dehydration processes are still not completely understood due structure instability after dehydration.