A method of measuring the instantaneous uncompensated resistances R(u) and double layer capacitances C-dl during the course of a dynamic faradaic process is presented. Values of R(u) and C-dl may be extracted at each step in the applied potential waveform. This contrasts with the approximate method of measuring R(u) and C-dl in supporting electrolyte devoid of analyte and with steady state measurements of impedance spectroscopy. The new method opens up possibilities of studying the dependence of capacitance on potential, species concentration, and time, e.g. for a reaction which is accompanied by changes in the electrode surface. Since measurements of R(u), C-dl and the faradaic reaction are contained within the same experiment, correction for IR drop and charging currents becomes possible even when there are large fluctuations in the electrolyte composition or the geometry of the electrochemical cell. The basic relationships for correction of IR drop and charging currents have been known for over 20 years [J.C. Imbeaux and J.M. Saveant, J. Electroanal, Chem., 44 (1973) 169; L. Nadjo, J.M. Saveant and D. Tessier, J. Electroanal. Chem., 52 (1974) 403]. However, it appears that their application to the general case of an arbitrary potential waveform comprising a sequence of potential steps has largely been neglected [K.B. Oldham and J.C. Myland, Fundamentals of Electrochemical Science, Academic Press, San Diego, 1994, p. 398]. A method for these corrections is therefore presented.