We report here our study on development of adsorbents suitable for capturing CO2 from synthesis gas (syngas) at high temperatures (>100 °C). Our adsorbents are based on double salts of MgCO3 and K2CO3 in which samples with different ratios of Mg:K were prepared by the wet mixing method of magnesium nitrate and potassium carbonate. The adsorbents were characterized by X-ray diffraction analysis, thermogravimetric analysis (TGA), and N2 adsorption and desorption at 77 K. The morphology of the samples was observed with scanning electron microscopy (SEM). CO2 adsorption experiments were performed at four temperatures, 300 °C, 350 °C, 375 °C, and 400 °C, which correspond to the operating temperature range of gases exiting a typical water gas shift reactor and entering a gas turbine in the Integrated Gasification Combined Cycle (IGCC) process. The CO2 adsorption amounts were 1.65 wt%, 8.47 wt%, 8.55 wt%, and 0.63 wt% respectively at a CO2 partial pressure of 100 kPa. These adsorbents also exhibited excellent cycling stability both in temperature swing adsorption and pressure swing adsorption operation. The kinetics of CO2 adsorption at different temperatures was obtained by using the linear driving force (LDF) model and a diffusion activation barrier of 21 kJ/mol was also inferred.