Photoelectrochemical and electrochemical reduction of CO_2 into organic chemicals is promising for directly or indirectly transforming solar energy into chemical energy for further utilization. However,research on the electroactive species in these processes has been rather limited. In this work, we investigated possible electroactive species(CO_2 or HCO_3~– ) involved in the electrochemical reduction of KHCO_3 at elevated temperatures without CO_2 bubbling. The results showed that CO, CH_4, and C_2H_4 were produced after electrochemical reduction of 3.0 mol/L KHCO_3 at elevated temperature on a Cu electrode even without CO_2 bubbling, although their faradaic efficiencies were low(< 6 %). Measurements for CO_2 generation from the decomposition of HCO_3~– showed that elevated temperature and high HCO_3~– concentration strongly promoted this process. These results suggested that the in-situ produced CO_2 from the decomposition of HCO_3~– was probably the electroactive species in the electrochemical reduction of HCO_3~– without CO_2 bubbling. Changes of the Gibbs free energy, rate constant, and activation energy of the decomposition of HCO_3~– into CO_2 were also investigated and calculated from the experimental data. Photoelectrochemical and electrochemical reduction of CO_2 into organic chemicals is promising for directly or indirectly transforming solar energy into chemical energy for further utilization. However, research on the electroactive species in these processes has been rather limited. The results showed that CO, CH_4, and C_2H_4 were produced after electrochemical reduction of 3.0 mol / L KHCO_3 at elevated temperature on a Cu electrode even without CO_2 or HCO_3 ~ -) involved in the electrochemical reduction of KHCO_3 at elevated temperatures without CO_2 bubbling. CO_2 bubbling, although their faradaic efficiencies were low (<6%). Measurements for CO_2 generation from the decomposition of HCO_3 ~ - showed that elevated temperature and high HCO_3 ~ - concentration strongly promoted this process. These results suggested that the in-situ produced CO_2 from the decomposition of HCO_3 ~ - was probably the electroactive species in the electrochemical reduction of HCO 3 ~ - without CO 2 bubbling. Changes of the Gibbs free energy, rate constant, and activation energy of the decomposition of HCO 3 - into CO 2 were also investigated and calculated from the experimental data.