Carbon microfibers (CMF) from polymeric substrates, such as polyacrylonitrile (PAN) have been proposed in the literature as potential CO2 adsorbents due to their high porosity and high content of nitrogen. They are flexible materials, whose adsorption properties may be modified by tailoring synthesis conditions, aiming at a given target, e.g. enhance CO2 selectivity over other gases. The response surface is a well-known empirical method to assess the effect of two or more variables on the behavior of one response variable. In this work, we examined the impact of carbonization temperature and PAN solution concentration (variables) on the response variables BET specific surface area and adsorbed concentration of CO2 and CH4. It has been found that BET area and CH4 adsorbed concentration are directly affected by the temperature of carbonization of fibers, whereas PAN solution concentration has much less impact on these two responses. However, for CO2 adsorbed concentration as response variable, both carbonization temperature and PAN solution concentration play a major role. Optimal conditions were found for carbonization temperature at 800 degrees C and PAN concentration between 9 and 10%.