H-2-rich (e.g. 75%) H-2-Ar mixtures are advisable (A) to run the TPR measurement at a practically constant H-2 concentration, (B) to keep an identical temperature throughout the specimen and a stable zero line, (C) to maintain the reactants concentrations uniform in the specimen, and (D) to generate data comparable with ones achieved with pure H-2 as reductant. Still waiting for experimental evidence, merits B and C should be essential if catalyst specimens and thermal effects are large. Merits A and D were borne out by J.Z's own TPR studies on calcined NiO/Al2O3 containing free NiO and combined NiOxAl(2)O(3), reported elsewhere. Water vapor, 75% H-2-Ar + 0.6% H2O, retarded considerably the reduction of NiO/Al2O3. The effect of H2O should be borne in mind, particularly in TPR-based catalyst morphology studies.