The equation for ideal capacitive behavior of a metal/solution interface was modified in such a way that it allows direct extraction of double-layer capacitance (C-dl) values as a function of the frequency from complex plane plots exhibiting nonideal behavior. Experimental impedance data verified by the Kramers-Kronig (K-K) transforms were presented for a mild steel/hydrochloric acid interface in the absence and presence of an organic additive. The results revealed that the modified equation works well for real systems.