Formation constants of metal complexes have been determined from pH titration data by use of a new method of calculation. In this method the sum of the squared fesiduals of all the mass balance equations is minimised with respect to the unknown parameters, the partial derivatives being evaluated analytically. The shifts of the adjustable parameters, calculated from a least-squares cycle, are optimised so that the miximum decrease of the error square sum is obtained for that cycle. A computer programme, based on this method, has been written and its performance has been compared with those obtained using other programmes previously described.