In our last paper (Hu, L.; Adeyiga, A. A. 1997, 36 (6), 2375), we reported the extraction of formic acid from the reaction of sodium formate with carbon dioxide by organic tertiary amine. The process can be described by the reactions HCOONa(w) + CO2(g) + H2O(w) === NaHCO3(s,w) + HCOOH(w) and HCOOH(w) + NR3(o) === HCOOHNR3(o). We defined the above process as a decomposition-extraction process. The studies investigated the mechanism of the decomposition-extraction process. By engineering analysis of the process, it was found that the mass transfer of carbon dioxide from the gas phase to the aqueous phase (the aqueous solution of sodium formate or calcium chloride) proceeded essentially through the organic phase (organic tertiary amine with diluent 2-ethylhexyl alcohol) rather than directly. Then carbon dioxide in the organic phase reacted with salt in the aqueous solution to form MHCO3 and complex NR3-HOOCH. The organic phase was both the extractant of formic acid and the transporter of carbon dioxide. This mechanism has been identified through our experimentation for the first time.