Effect of the compactness of the lithium chloride layer formed on the carbon cathode on the electrochemical reduction Of SOCl2 electrolyte in U-SOCl2 primary battery was investigated using ac-impedance spectroscopy and potentiostatic current transient technique. From the facts that the straight lines of the Nyquist plots of the ac-impedance spectra and the peak-like runs of the plot of It(1/2) versus log t were observed from the pure carbon cathode, it was suggested that the porous layer of lithium chloride deposited on the pure carbon cathode was relatively compact enough to strongly impede the diffusion of SOCl2 through it, and hence the rate-cont rolling step for overall SOCl2 reduction is changed from the 'interfacial reaction between the pure carbon cathode and electrolyte' to the 'diffusion of SOCl2 through the porous lithium chloride layer'. On the other hand, any of the straight lines of the Nyquist plots of the ac-impedance spectra and of the peak-like courses of the plot of It(1/2) versus log t can not be found in the Co-phthalocyanine (Pe)-incorporated carbon cathode. Thus, it was concluded that the porous layer of lithium chloride formed on the Co-Pe-incorporated carbon cathode was relatively porous enough to considerably facilitate the diffusion Of SOCl2 through it, and hence the overall reduction rate of SOCl2 is governed by the 'interfacial reaction between the Co-Pe-incorporated carbon cathode and electrolyte' throughout the whole discharge of the Li-SOCl2 batteries.