In studies of metal ion complexes formed in solution, the magnitudes of equilibrium constants existing between the various species are usually sought. Ideally, these constants are obtained from relationships between activities of each of the species present under equilibrium conditions. Often, however, the activities are unknown and concentrations instead of activities must be used. These latter concentration equilibrium constants, as distinct from activity equilibrium constants, are a function of the ionic strength of the medium and consequently must be evaluated from sets of measurements all made at the same ionic strength. Most studies to date on concentration equilibrium constants, such as stability constants of metal ion complexes in aqueous media, have arbitrarily chosen perchlorate or nitrate media to maintain the ionic strength constant, with the assumption that this added perchlorate or nitrate electrolyte is noncomplexing toward the metal ion being studied. Furthermore, as a consequence of this attitude, little quantitative information on the actual stability of perchlorate or nitrate complexes is available. In this work, consideration is given to errors and effects introduced into results obtained for concentration stability constants in making the assumption that the added electrolyte is noncomplexing with special reference toward the commonly used polarographic method for determining stability constants. Results are then given for stability constants of perchlorate, nitrate, and chloride complexes of thallium (I), obtained by the polarographic method, using fluoride solutions as the noncomplexing medium, to show that perchlorate and nitrate may not always be the best choice as an inert electrolyte and that thought at least should always be given to possible alternatives.