The ionization balance of the circumstellar gas around Herbig Ae/Be stars is studied. The model used is based on the results obtained by Catala & Kunasz (1987) for the wind formation region of the typical Herbig Ae/Be star AB Aur. Large brightness variations of some stars are assumed to be due to their obscuration by clumps in the shells, and the gas ionization in the clumps is also considered. It is shown that the source of ionization of gas is the radiation of the stellar photosphere and chromosphere, and the main processes that determine the ionization structure of the shells are photoionization, photorecombination and dielectronic recombination. The column density of Na I and Ca II, the central optical thickness and the equivalent width of the circumstellar Ca II K and Na I D₂ absorption lines are estimated and their dependence on the model parameters is examined. It is found that the clumps are the places where sufficiently strong variable lines may originate. The equivalent width of the lines are mainly determined by the gas to dust ratio and the element depletion in the clumps. The lines could be observed with the current large telescopes and give information on the nature of the clumps being probably the places of planet formation. The model is used to interpret two episodes of the spectral line variations observed for AB Aur and HR 5999.