The analysis of random errors is discussed for the equilibrium constants of very stable metal complexes, determined through competitive potentiometric methods. Prior to this analysis the calculation of standard deviations both for overall and stepwise equilibrium constants is considered. Clues are given as to how a system can be properly characterized. To illustrate the error analysis, the following systems were selected: Cu(II) andL-2,6-diaminohexanoic acid (L-lysine); Cu(II) and ethylene diaminotetraacetic acid (EDTA); and Cu(II), EDTA and 1,2-dihydroxybenzene (catechol).