The knowledge of the thermodynamic parameters related to the hydration of organic solid phases is paramount to exert a very good control over this transition during industrial production or storage. To study this phenomenon, the citric acid was chosen as a model compound. By an original combination of gravimetric and thermal analyses (dynamic vapor sorption and discontinuous isoperibolic thermal analysis, respectively) with structural determination (prototype of in situ X-ray diffractometer) the hydrate/anhydrate transition of the citric acid was thermodynamically characterized by determining: (i) the temperature associated to the peritectic transition <CA, 1H2O> ↔ <CA> + H2Ol, as well as (ii) the minimum relative humidity of hydration, and (iii) the critical relative humidity of deliquescence versus temperature. All these studies lead to the proposition of phase diagram between citric acid and water as a function of temperature and partial vapor pressure of water.