Glassy carbon electrode (GCE) was activated with four different electrochemical activation process: (1) by applying a fixed potential of +1.7 V for 400 s (FP-AGC); (2) by cycling the potential between +2.0 and −0.3V for 15 cycles at 0.1 V/s (CP-AGC); (3) by applying a fixed potential of +1.7 V for 400 s followed by cycling the potential between +2.0 and −0.3V for 15 cycles at 0.1 V/s (FC-AGC) and (4) by applying +1.7 V for 400s followed by -1.0 V for 60s (DP-AGC). All four activated GCEs showed catalytic activity and reversibility toward the oxidation of both hydroquinone (HQ) and catechol (CT) in 0.1 M phosphate buffer solution (PBS, pH 7.0). However, a remarkable signal enhancement of HQ and CT was obtained at DP-AGC, which is considered as optimized activated GC. By using this optimized AGC we have developed a highly sensitive and selective electrochemical method for the determination of HQ and CT simultaneously. The redox responses from the mixture of HQ and CT were easily resolved at DP-AGC. The detection limits for HQ and CT were calculated to be 15.0 and 10.0 nM respectively. The optimized AGC was successfully examined for real sample analysis with tap water and it showed a stable and reliable recovery data with reproducibility.