Equilibrium binding studies have been carried out by spectrofluorometric precision titrations on FITC-Na,K-ATPase and employing the styryl dye RH-421 to obtain equilibrium constants and stoichiometric coefficients together with information related to competition between different binding equilibria. A new interpretation concerning the assignment of spectral properties and cation complex formation equilibria, as well as the involvement of conformational transitions, is suggested, based on a differentiation between selective and unselective alkali ion binding. The kinetics of K+ binding to the FITC-enzyme have been studied by employing a new microvolume technique consisting of flash photolysis of caged-K+.