The interfacial properties of two hydrous oxides of manganese, namely an amorphous variety and a poorly crystalline synthetic birnessite, in different electrolyte media, have been studied by using a batch potentiometric titration procedure with a 72-h equilibration period. The data obtained by this procedure are more relevant to a natural water system than those obtained by the rapid titration method as reported earlier [J. Colloid Interface Sci. 131, 103 (1989).]. Although the points of zero charge do not show any major variations, the surface dissociation and complexation (with Na+ in NaCl) constants differ with the method of titration. Both of the samples are characterized by their high surface charge (low Delta pK(a)) and surface potential, and as a result, complexation takes place possibly within the inner helmholtz plane. It is concluded that the triple-layer model is not applicable to this colloid system and that a basic Stern model would be more suitable in this case.