A thorough study of the adsorption behavior of shales to methane (CH4) and carbon dioxide (CO2) is critical for carbon dioxide sequestration in shale gas reservoirs and enhanced shale gas recovery. This paper discusses the results of an adsorption study of CH4 and CO2, in single gas environment, on a set of shale samples taken from the Sichuan and Ordos Basins. The results indicated that shale exhibit higher affinity to CO2 as compared to CH4 under similar pressure and temperature conditions, and the preferential adsorption ratio of CO2 over CH4 varies between 1.66 and 8.32. Furthermore, the experimental data were modeled using Langmuir, Dubibin-Astakhov (D-A), Dubinin-Redushckevich (D-R) and Ono-Kondo models. The accuracy of the models in quantifying CH4 and CO2 adsorption on shale was compared using an error analysis technique. For CH4, both of Langmuir and D-A models performed satisfactorily with comparable accuracy. For subcritical CO2, the performance of the D-A model is the best. However, for supercritical CO2, Ono-Kondo model fitted the experimental data the best, which is the proposed model to use. The results of this study can provide the basis for the estimation of the CO2 sequestration capacity in shale formations.