ABSTRACTWe have developed a QCM (Quartz Crystal Microbalance) based method for direct gravimetric determination of water adsorption on PuO₂ surrogate surfaces, especially CeO₂, under conditions representative of those in a typical PuO₂ storage can. In this application, the method of transduction of the QCM relies upon the linear relationship between the resonant frequency of piezoelectrically active quartz crystals and the mass adsorbed on the crystal surface. The spurious effect of high temperatures on the resonant frequency of coated QCM crystals has been compensated for by modeling the temperature dependence of the frequency response of the surrogate coated-QCM crystal in the absence of water. Preliminary results indicate that water is readily adsorbed from the vapor phase into porous metal oxide structures by capillary condensation, an observation that may have important ramifications for water uptake within the packed powder beds that may obtain in PuO₂ storage cans.