A package of new isotherm equations aimed at examination of pore structure of sub- and microporous materials with respect to a surface heterogeneity is proposed. One considers adsorption of small nearly spherical molecules in irregular pores of molecular size. A generalized BET theory is exploited together with a thermodynamic description of the process and handling restrictions for multilayer adsorption (LBET approach). Realistic multivariant relationships linking a surface energy distribution with the pore size are proposed, and exponential distribution of the pore size is accepted. As a result, the heterogeneous adsorption equations (uLBET formulas) are derived, dedicated for irregular microporous systems. It was shown that they may be well-approximated with analytical BET-like formulas (the models of the LBET-type), enabling fast multivariant examination of real microporous materials. Such an examination provides complementary information on adsorption mechanisms, thus, allowing more reliable evaluation of the system parameters. Results of the new model's application to an empirical system (nitrogen adsorption on an activated carbon) are shown.