The electrocatalytic reduction of CO2 has been studied extensively and produces a number of products. The initial reaction in the CO2 reduction is often taken to be the le formation of the radical anion, CO2 center dot-. However, the electrochemical detection and characterization of CO2 center dot- is challenging because of the short lifetime of CO2 center dot-, which can dimerize and react with proton donors and even mild oxidants. Here, we report the generation and quantitative determination of CO2 center dot- in N,N-dimethylformamide (DMF) with the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM). CO2 was reduced at a hemisphere-shaped Hg/Pt ultramicroelectrode (UME) or a Hg/Au film UME, which were utilized as the SECM tips. The CO2 center dot- produced can either dimerize to form oxalate within the nanogap between SECM tip and substrate or collected at SECM substrate (e.g., an Au UME). The collection efficiency (CE) for CO2 center dot- depends on the distance (d) between the tip and substrate. The dimerization rate (6.0 x 10(8) M-1 s(-1)) and half-life (10 ns) of CO2 center dot- can be evaluated by fitting the collection efficiency vs distance curve. The dimerized species of CO2 center dot-, oxalate, can also be determined quantitatively. Furthermore, the formal potential (E-0 ') and heterogeneous rate constant (k(0)) for CO2 reduction were determined with different quaternary ammonium electrolytes. The significant difference in k(0) is due to a tunneling effect caused by the adsorption of the electrolytes on the electrode surface at negative potentials.