SBFO is a commercially available fluorescent indicator for the determination of Na+ concentrations. To investigate the reversible reaction of Na+ with this indicator in the ground and excited states, steady-state and time-resolved fluorescence measurements were performed. Global compartmental analysis was used to describe the fluorescence decay surfaces yielding the following values for the rate constants at room temperature in aqueous solution at pH 7.2: k(01) = 5 x 10(8) s(-1), 2 x 10(7) M-1 s(-1) < k(21) < 3 x 10(8) M-1 s(-1), k(02) = 5 x 10(8) s(-1), k(12) = 1 x 10(8) s(-1). k(01) and k(02) denote the respective deactivation rate constants of the Na+ free and bound forms of the indicator in the excited state. k(21) represents the second-order rate constant of association of Na+ with SBFO in the excited state, whereas k(12) is the first-order rate constant of dissociation of the excited Na+-indicator complex. The effect of the reversible excited-state reaction between Na+ and the indicator on the fluorimetric determination of K-d can be neglected.