The pervasive concept of the cause of the potential occurring across a semipermeable membrane intervening between two ionic solutions is called the membrane theory; hence, this potential is called the membrane potential. Although almost nobody has doubted its validity, research results defying it have been continuously reported by a small number of researchers. They have claimed that the cause the potential lies in the adsorption of ions onto adsorption sites, which is the adsorption theory. One such research report by G. Colacicco (Nature 207 (1965) 936) was employed for our experimental work reported in this paper in order to examine the validity of the membrane theory and the adsorption theory. The results we obtained are in conflict with the membrane theory but appear to be in full agreement with the long dismissed adsorption theory. This paper urges the reexamination of the membrane theory and the reconsideration of the adsorption theory from a nonbiased standpoint. (C) 2004 Elsevier Inc. All rights reserved.