The adsorption of CO2 on commercial activated carbon WG12 and WG12 modified by ZnCl2 and KOH was investigated using a high pressure Sievert's apparatus. The experimental data were analyzed using Langmuir, Freundlich, Sips, Toth, Unilan, Ftitz-Schlunder, and Redlich-Peterson equations. An error analysis was performed to examine the effect of using different error criteria for the isotherm parameters determination. Five error analysis methods were used: the sum of the squares of errors, the hybrid fractional error function, the average relative error, the Marquardt's percent standard deviation, and the sum of the absolute errors. The Sips isotherm provided the best quality of fitting for all the experimental data. Error function analysis found that sum of the squares of the errors and hybrid fractional error function provided the best overall results.