Thermal analysis has shown that contrary to the case of Zn2+, considerable amounts of water enter in HFO (hydrous ferric oxide), HSO (hydrous stannic oxide), and HMO (hydrous mixed oxide) when Co2+ is adsorbed. A large amount of Co2+ and Zn2+ in all the oxides cannot participate in isotopic exchange, seemingly because of being trapped inside the micropores, a fact hindering oxide capacity determination by isotopic dilution. The % adsorption-pH curves for tracer and 10(-3) M transition metal chloride solutions show a general preference to Zn2+ over Co2+ by all the oxides, in agreement with thermal analysis data, Large shifts of these curves to the left occur with tracer experiments on many samples on adding 0.1 M NaCl, probably due to Cl- adsorption accompanied by a change in the adsorption mechanism. Conclusions from log K-d-pH plots may be erroneous, probably due to changes in the activity coefficients of surface species even at sufficiently low surface coverages. More adsorption of Co2+ and Zn2+ generally occurs on the oxide of lower pH(PZC) (higher acidity),