The recent interest in testing Mitchell's chemiosmotic hypothesis [1, 2] has stimulated the s tudy of t ransmembrane electrochemical p ro ton gradients in various biological systems. The main postulate of the chemiosmotic hypothesis is that in energy conserving membranes there are electrogenic p ro ton pumps, driven by electron transport , which builds up a p ro ton electrochemical potential difference, A~I H. This potential is postulated to be the driving force for ATP synthesis by a reversal of an ATPase pro ton pump. The p ro ton electrochemical potential difference, A/)H (or Zip as it is called by Mitchell [2] ), is divided into electrical and concentra t ion terms according to the relation: