Abstract The coulostatic method has been successfully used in the study of electrode processes. In the literature, the technique is explained on the premise that the step-charge is transferred to an ideal interfacial capacitance. Here, for the first time, we have generalized the coulostatic test method by considering a non-ideal interfacial capacitance modeled by a constant phase element (CPE), which agrees with the relaxation processes observed experimentally. The methodology proposed allows one to determine the parameters of a modified Randles circuit consisting of a resistor in series with the parallel combination of a CPE and another resistor. A set of useful expressions for the transient response to a rectangular current pulse is derived using fractional calculus and circuit theory. In addition, we analyze the width of the pulse required to guarantee an efficient coulostatic approach, that is, the total charge associated with the current pulse is transferred to the CPE. Both the method proposed and the impedance spectroscopy (IS) technique (for comparative purposes) are used to study a two-electrode setup consisting of two platinum (Pt) electrodes in contact with a phosphate-buffered saline (PBS) solution.