The simultaneous voltammetric determination of catechol (CC) and hydroquinone (HQ) has been achieved at a mesoporous carbon CMK-3 modified electrode in phosphate buffer solution (pH 7.0). At the electrode both CC and HQ can cause a pair of quasi-reversible and well-defined redox peaks and their peak potential difference increases. In comparison with multi-walled carbon nanotubes (MWCNTs) and Vulcan XC-72 carbon modified electrodes the CMK-3 modified electrode shows larger peak currents and higher adsorbed amounts for the two dihydroxybenzene isomers. This is related to the higher specific surface area of CMK-3. Under the optimized conditions, the linear concentration ranges for CC and HQ are 5×10−7 to 3.5×10−5 M and 1×10−6 to 3×10−5 M, respectively. In the presence of 5μM isomer, the linear concentration range of CC (or HQ) is 5×10−7 to 2.5×10−5 M (or 5×10−7 to 2.0×10−5 M). The sensitivity for CC or HQ is 41AM−1 cm−2 or 52AM−1 cm−2, which is close to that without isomer. The detection limits (S/N=3) for CC and HQ are 1×10−7 M after preconcentration on open circuit for 240s.