Using the 100% relative‐humidity technique for controlling the mole function of water vapor, the Napier (relaxation) frequency of oxygen has been measured as a function of the mole fraction of various water vapors, consisting of pureH2O, pureD2O, and three mixtures thereof. PureH2Ogives, as is well known, a quadratic inH, the mole fraction; pureD2Ogives a linear relation down to 0.7×10−3. The mixtures show a linear relation above 1.5×10−3and a curvilinear approach to zero below that. Calculation indicates that the heterogeneous molecule HDO is far more effective as a thermal equilibrator for oxygen than either of its homogeneous parents. Quantitatively, with (f/p)min cps per atm andhin mole fraction ×103, the results are:100%H2O:(f/p)m = 7+183h+132h2;50%D2O:(f/p)m = −664+1100h;80%D2O:(f/p)m = −490+908h;92%D2O:(f/p)m = −250+585h(aboveh= 1.3);92%D2O:(f/p)m = 7+275h+100h2(belowh= 1.3);99.8%D2O:(f/p)m = −85+385h.