Microporous carbon films obtained from cellulose and then chemically modified (oxidation and loading with metal cations, nine samples altogether) are investigated to evaluate their porosity. The materials obtained are microporous; moreover, the majority of micropores possess the same diameter. The results of the calculation of the average pore diameters using different methods based on the analysis of low-temperature nitrogen adsorption data and the measurement of the benzene adsorption isotherm and enthalpy are presented and analyzed. The results obtained with the DFT and HK methods are similar. It is concluded that a good correlation between pore diameters obtained from DFT analysis (for nitrogen adsorption) occurs for those calculated from benzene calorimetry data using the Everett and Powl method and those calculated by applying a recently developed equation.