Enzymatic carbon fixation is one of the most interesting processes in CO2 sequestration. A number of decarboxylases can catalyze the reversible decarboxylation reaction in vivo while they can enhance the carboxylation reaction in vitro. The Bio-Kolbe-Schmitt reaction is the carboxylation of phenols and CO2 by hydroxybenzoic acid decarboxylases to form hydroxybenzoic acids under mild conditions that require an alkaline environment. It remains unclear how carboxylation can improve carbon fixation. In this study it was described that the main problem in switching from decarboxylase to carboxylase is through thermodynamic calculation and determination of the enzyme properties, CO2 concentration and other related species in the reaction system such as HCO3-. We have found that the concentration of CO2 in aqueous solution is the main factor in achieving a high yield of CO2 fixation. To get enough CO2 (aq) to promote carboxylation by decarboxylase in an alkaline environment, a high concentration of HCO3- is required. The highest CO2 fixation yield of 30% was achieved by reacting under 0.2 MPa CO2 in 2.7 M KHCO3 buffer for 15 minutes. Under the optimal reaction conditions, the carboxylation of catechol by 2,3-DHBD_Ao (2,3-dihydroxybenzoic acid decarboxylase from Aspergillus oryzae) achieved a good “turnover frequency” (TOF) of 47 min & supmin; ¹. This study shows that the CO2 (aq) concentration is a crucial parameter for increasing the CO2 fixation yield in a homogeneous reaction under alkaline conditions.