On the other hand, the electrooxidation processes of APAP and AA in mixtures were also investigated when the concentration of one species changed and the other was kept constant, and the results are shown in Fig. 5. Examination of Fig. 5A shows that the peak current of APAP increased with increasing APAP concentrations, whereas the concentration of AA remained constant (100.0 μM). Similarly, as shown in Fig. 5B, keeping the concentration of APAP constant (35.0 μM), the oxidation peak current of AA was positively proportional to its concentration; whereas the oxidation peak current of APAP did not change. If the concentrations of APAP and AA increased synchronously, the peak currents at the SWCNT/CCE increased accordingly, as shown in Fig. 6. It can be seen that the peak currents for the two analytes increased linearly in the wider ranges with their concentrations (insets of Fig. 6). The voltammetric responses of the SWCNT/CCE toward the simultaneous determination of APAP and AA are listed in Table 1.