Co3O4 powders prepared by thermal decomposition of nitrate (TDN) and by precipitation in aqueous solution (PAS) have been investigated in relation with the zinc-ion adsorption (ZIA) method of in situ measurement of specific areas at the solid-liquid interface. The ZIA method seems feasible with dry TDN powders, while high solubility and cation exchanges hinder the method with hydrous PAS powders. The ZIA surface areas of dry Co3O4 are approximately 1/2 the BET surface areas.