To produce natural gas (methane) and simultaneously sequester CO2 in unconventional geologic reservoirs such as gas shale, coalbeds and so on, it is necessary to understand the adsorption behavior of methane and CO2 in these reservoir formations. In this article, adsorption behavior of methane and CO2 on shale samples from Gondwana Basin and KG Basin of India are studied. Adsorption experiments are conducted on as-received shale samples from these basins at a temperature of 313 K to a maximum equilibrium pressure of approximately 9 MPa for methane and 6 MPa for CO2. The methane and CO2 adsorption data are applied to test the applicability of Langmuir, Dubinin-Polanyi, BET, and Ono-Kondo models. A comparison of these models is performed using linear and nonlinear Chi-squared methods. It was observed that Dubinin-Astakhov equation was the most accurate adsorption isotherm model for adsorption of methane and CO2 on tested shales. Further, the better fitting by Dubinin-Polanyi equation over BET, Langmuir, and Ono-Kondo models suggest that the mechanism of volume filling may be applicable during the adsorption of methane and CO2 on shales. The preferential adsorption of methane and carbon dioxide on Pakur, KG, and Salanpur shales were investigated. The Pakur shale had CO2:CH4 adsorption ratio ~1. © 2019 American Institute of Chemical Engineers Environ Prog, 38: 13222, 2019. © 2019 American Institute of Chemical Engineers