A series of hydration experiments of the Ruddlesden-Popper phase PrSr3Co1.5Fe1.5O10-delta with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to delta approximate to 1 . Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO2 free conditions pure hydrates and oxide hydroxides are formed. In CO2-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The I-centered PrSr3Co1.5Fe1.5O5(OH)(2)center dot 1H(2)O achieves a highly expanded c-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr3Co1.5Fe1.5O8(OH)(2), at 70 degrees C with a primitive orthorhombic unit cell. Upon heating above 170 degrees C, an I-centered product is formed for which further dehydroxylation occurs at around 400-500 degrees C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO](RS) rock salt layer of the monoclinic hydrate.