Adsorption on a heterogeneous surface of flexible molecules composed of a number of mers, like n-alkanes for instance, is strongly affected by topography of surface adsorption sites. Therefore, fitting to experimental adsorption isotherms, theoretical expressions developed for multisite-occupancy adsorption on heterogeneous surfaces characterized by random and patchwise topography should make it possible to distinguish whether a studied solid surface is characterized by random or by patchwise topography. For certain fundamental but, also practical reasons, we took into consideration carbonaceous adsorbents. Following the general approach to multisite-occupancy adsorption published recently by Rudzinski and Everett, we have developed here a generalized form of the Dubinin-Astakhov isotherm equation, taking into account surface topography and the interactions between the adsorbed molecules. The developed isotherm equation has been used successfully by us to study the nature of surface topography of a number of carbonaceous adsorbents.