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Observations on the activity of calcium oxide powder for heterophase hydrogen-isotope exchange processes of the type : 2H,(g)+'HO-(s)~2HLH(g)+2HO-(s) 'H2(g) + 2 2HO-(s) and/or 2H2(g) + 2 'HO-(s) after outgassing in uacuo at progressively higher temperatures, &, provide a sensitive monitor for variations in the number of surface hydroxyls capable of H/D exchange using 573 or 673 K as the fixed reaction temperature, T,,. Observations on activity for heterophase oxygen-isotope exchange with T,, = 623 K reveal a strong similarity in dependence upon cg to that for heterophase hydrogen exchange. This similarity indicates that l80,(g) + IsOH-(s) 160'80(g) + 180H-(s) rat her than is the dominant pathway to heterophase oxygen-isotope exchange at T,, = 623 K after cg in the range 573-973 K. Information on the extent to which surface 02-(s) or OH-(s) contributes to 160/180 exchange at T,, = 698 or 773 K is sought from a sequence of measurements at each qx,after Tg in the range 623-1 123 K. Delayed onset of the homophase oxygen-isotope equilibration process '802(g) + '602-(s) e '60'80(g) + 1802-(s)'802(g) + 1602(g) e 2'60180(g) until after the CaO powders had experienced cg 2 973 K is observed and attributed to a need for most surface hydroxyls to be removed before defect sites required for this exchange become available.

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