The growth kinetics of the lithium chloride layer formed on the porous carbon cathode during the discharge of a Li-SOCl2 primary battery was investigated using galvanostatic discharge experiment and ac impedance spectroscopy. From the analysis of the high-frequency arc in a Nyquist plot of the ac impedance spectra, it was observed that, as discharge progressed, there appeared a low to high transition in the average growth rate of the primary film over a narrow range of depth of discharge, regardless of discharge current. The inclined line in the intermediate frequency range associated with the secondary layer revealed that there also existed a transition of the straight line with a phase angle higher than 45degrees to the convex line with a phase angle lower than 45degrees with progressing depth of discharge, regardless of discharge current. A conventional transmission line model was modified to numerically analyze the ac impedance spectra for the noncylindrical pores comprising the porous secondary layer based upon a series combination of position-dependent resistance and capacitance in value. From the coincidence in shape of the impedance spectra experimentally measured with those theoretically calculated in the intermediate frequency range, it was concluded that there occurred a transformation of the conically shaped pores to warhead-shaped pores comprising the secondary layer with progressing depth of discharge. The compactness of the secondary layer increased with progressing depth of discharge below and above the transition depth of discharge. The transition cathodic overpotential at which both the transition depth of discharge for the primary layer and the secondary layer just appeared was measured to be about between 360 and 400 mV, regardless of discharge current. (C) 2003 The Electrochemical Society.