Differential capacitance versus frequency curves simulated for different equivalent circuits representing different cases of anion adsorption have been analyzed. The analysis showed that in all cases with an ideal, homogeneous electrode surface, the real Value of the differential capacitance, representing the sum of the double layer and adsorption capacitances (independent of frequency) could be obtained only at frequencies lower than about 2 Hz. In the case of real surfaces, presented by introduction of a constant phase element instead of a double layer capacitance, differential capacitance versus frequency curves were found to be more complex, with no possibility to obtain values of the double layer capacitance at high and adsorption capacitance at low frequencies, since the differential capacitance was found to depend on frequency in the whole range of frequencies (10(-4)-10(5) Hz). Chloride adsorption onto the Ag(111) face in 0.01 M NaCl solution was found to follow the approach initiated in this work by introducing a constant phase element instead of the double layer capacitance. It is shown that impedance diagrams deviate from the 'ideal double layer behavior' and that differential capacitance versus potential curves depend on frequency. It is confirmed by the analysis of differential capacitance versus frequency curves that this is the consequence of adsorption of chloride anions presented by adsorption resistance and capacitance connected in series, as well as of inhomogeneity of the real single crystal face.