Taking into account that during polarographic measurements at a constant ligand to metal ratio and gradually increasing pH value, the slope of ΔEp vs. pH plot is sometimes used for identification of the major complex species in the solution, characteristics of the mentioned relationship were studied in more detail. Assuming reversibility of the electrode process, kinetic lability of all the complexes and absence of possible complications, such as surface activity of dissolved species, it was demonstrated (by model calculations) that the shape, slope(s) and position of ΔEp vs. pH plot are highly affected by different factors. Even in very simple examples such as systems of HL-ML type numerous limitations (concerning the acceptable values of β1[L]t product, relative concentrations of free ligand and metal etc.) appear which can sometimes completely prevent the successful application of this diagnostic method for identification of the major complex species because experimental results obtained under usual laboratory conditions do not meet the criteria. If, however, the mentioned limitations are ignored completely erroneous results could be obtained.