The present study was done primarily to compare cation-ATPase dephosphorylation kinetics with a Cl−-ATPase's dephosphorylation kinetics because of the paucity of information in this area. Utilizing a proteoliposomal preparation containing Cl−-ATPase from Aplysia gut, it was demonstrated that dephosphorylation of this P-type ATPase was absolutely dependent upon Cl−. Adenosine triphosphate (ATP) concentrations directly stimulated dephosphorylation of Cl−-ATPase in the presence of increasing concentrations of Cl−. It was also shown that the calculated rate constant for E1-P disintegration was 20/sec. This rate constant value approximated E1-P rate constant disintegration values for other electrogenic, uniport P-type ATPases. Therefore, it was concluded from these results that the Cl−-ATPase dephosphorylation kinetics did not differ greatly from cation-ATPase dephosphorylation kinetics. © 2002 Wiley-Liss, Inc.