Temperature-programmed desorption mass spectrometry (TPD-MS) measurements on [O-18]water-enriched copper sulfate pentahydrate (CuSO(4)(.)5H(2)(18)O) reveal an unambiguous occurrence of efficient oxygen isotope exchange between the water of crystallization and the sulfate in its CuSO4 solid phase. To the best of our knowledge, the occurrence of such an exchange was never observed in a solid phase. The exchange process was observed during the stepwise dehydration (50-300 degrees C) of the compound. Specifically, the exchange promptly occurs somewhere between 160 and 250 degrees C; however, the exact temperature could not be resolved conclusively. It is shown that only the fifth, sulfate-associated, anionic H2O molecule participates in the exchange process and that the exchange seems to occur in a preferable fashion with, at the most, one oxygen atom in SO4. Such an exchange, occurring below 250 degrees C, questions the common conviction of unfeasible oxygen exchange under geothermic conditions. This new oxygen exchange phenomenon is not exclusive to copper sulfate but is unambiguously observed also in other sulfate- and nitrate-containing minerals.