Oxygen isotope exchange during thermal dehydration of copper selenate pentahydrate was studied using the temperature-programmed decomposition mass spectrometry technique. The oxygen exchanges between the water of crystallization and the selenate in the solid phase. The results fairly indicate the preferable exchange between the oxygen atom of the most strongly bound water molecule and, at most, one specific oxygen atom in the selenate ion. The oxygen exchange probability was found to amount to 41.6%. Noteworthy is the fact that a small degree of oxygen exchange was also found to occur between selenate anion and liquid water. In addition, relating to previous studies on copper sulfate, the present study shows that both compounds possess identical oxygen exchange mechanisms in the solid state during their dehydration. (C) 2010 Elsevier By. All rights reserved.