The adsorption of binary mixtures on heterogeneous surfaces is studied by a cluster approximation (CA), based on the exact calculation of configurations on finite cells. The substrate is characterized by n types of sites, each with different adsorption energy. The process is monitored through the total and partial isotherms. The theoretical formalism is used to model experimental data of methane-ethane mixtures adsorbed on a template-synthesized carbon. The CA results are compared with the ones corresponding to the well-known Ideal Adsorbed Solution Theory (IAST). Even though a good fitting is obtained from IAST, it is found that CA is a more accurate model to estimate the binary data on the highly heterogeneous carbon sample. (C) 2016 Elsevier B.V. All rights reserved.