Time development of impedance spectra measured during passivation of lithium in 1.5 M LiAlCl4/SOCl2 is modeled quantitatively. The approach, which relates the changes in the high-frequency are with microstructural changes of the secondary film, is general enough to be applied to other passivating systems in which the impedance response varies considerably with time. Extensive correlation between the suggested approach and the well-known model of a parallel-plate capacitor is given. The low-frequency impedance are is associated with diffusion of Li vacancies and hopping electrons within the solid part of the passive film. The contentrations and diffusion coefficients of both carriers are estimated to lie in the range of 5000 ppm and 5 X 10(-11) m(2)/s, respectively. The apparent increase in "liquid electrolyte resistance" during passivation is explained in terms of the suggested modeling.