Composites (GrO@Cu-BTC) based on Cu-BTC and graphene oxide were synthesized by a solvothermal method for the separation of CO2/CH4 binary mixtures. The as-synthesized composites were then characterized. The isotherms of CO2 and CH4 on the as-synthesized materials were measured by the volumetric method. The isotherms and adsorption selectivities of CO2/CH4 binary mixtures were estimated on the basis of ideal adsorbed solution theory (IAST). The results showed that the composite 1GrO@Cu-BTC had a higher BET surface area and pore volume compared to the parent Cu-BTC. More importantly, its adsorption capacity for CO2 improved significantly in comparison with that of Cu-BTC, which was up to 8.19 mmol/g at 1 bar and 273 K. The dual-site Langmuir-Freundlich (DSLF) model was applied favorably for fitting experimental isotherm data of CO2 and CH4 adsorption on the samples. The predicted isotherms of the binary mixture based on IAST showed that CO2 was more favorably adsorbed than CH4 on the sample 1GrO@Cu-BTC. TPD showed that the desorption activation energy of CO2 on 1GrO@Cu-BTC was higher than that on Cu-BTC, indicating a stronger interaction between CO2 molecules and 1GrO@Cu-BTC. Thus, the CO2/CH4 adsorption selectivity of the composite 1GrO@Cu-BTC was significantly higher than that of Cu-BTC, namely, 14 at 1 bar, or 2.6 times that of Cu-BTC.