Various proposed methods for the determination of ligand purity and apparent equilibrium constant (K_app) in divalent cation buffer solutions are discussed by McGuigan et al. (2004) who show that the obstacles present in the other existing methods are overcome by the method of Luethi et al. (1997). However, this method involves laborious, iterative calculations and therefore it is not generally used.In the method of Luethi et al. (1997) the problem is to determine three parameters: ligand purity, K_app and the nonlinearity parameter in the Nicolsky-Eisenmann equation. This problem can be re-formulated using a constrained nonlinear least squares approach in which a single objective function of all three parameters is defined and minimised subject to the constraint that the ligand concentration is no greater than its nominal value. The Nicolsky-Eisenman equation relates the measured potentials in the buffer solutions to the values of the divalent cation. From Luethi et al. (1997), the concentration of the divalent cation may be written as a function of the total divalent cation concentrations together with the ligand purity and K_app.By substituting this equation for the divalent cation into the Nicolsky-Eisenman equation a single nonlinear regression model is obtained which directly links the measured potentials with the corresponding values of total concentrations of the divalent cation. The associated residual sum-of-squares function is then a single objective function which may be directly minimised with respect to all three unknown parameters, subject to the constraint on the ligand concentration. The method of Luethi et al. (1997) cannot be used at values of K_app greater than 0.1 mmol/l. However, if the concentration of the organic anion is known, the method can be easily modified to determine K .The binding of Mg²⁺ to malate illustrates both aspects.This automated approach has been implemented in an Excel spreadsheet using the freely-available Solver Add-In and it is driven by macros which have been written in Visual Basic. The programme is easy to use and, once the initial data have been entered and options selected, the solution is produced literally at the touch of a button. Given familiarisation, this programme makes the method proposed by Luethi et al. (1997) widely available for fast computation of the ligand purity and K_app of divalent cation buffer solutions and of K for divalent cations binding to organic anions of physiological importance (e.g. malate, citrate and aspartate). The programme which is demonstrated at this meeting is available for PC use from Jim Kay (jim@stats.gla.ac.uk).