Using the exact charge density of intrinsic point defects of metal oxides (MOs), the phenomenological electron theory of chemisorption, developed by Volkenstein, is applied to acceptorlike and donorlike chemisorption on MO thin films for the whole ξ region (where ξ=D/L_D, D is the film thickness, and L_D is Debye length). The experimental temperature and oxygen partial pressure dependence of the averaged electron concentration 〈n〉(p_O2,T)∝e^−E_A/kTp_O2^−m(T) for polycrystalline ZnO films are discussed on the bases of three different models: the Schottky‐defect model, the Volkenstein model for electronic equilibrium, and the comprehensive model for complete equilibrium. It turns out that a Schottky‐defect model that uses single‐crystal‐mass action constants will not yield the experimental high temperature limit (T=1000 K: E_A=1.6 eV, m=0.26). This limit is obtained using a higher averaged concentration of oxygen vacancies for polycrystalline films (due to the presence of grain boundaries). The comparison between the electronic and complete equilibrium shows that the screening of the surface charge through mobile positively charged oxygen vacancies has a tremendous reducing effect of about 30% of the surface potential in the temperature range considered.