Folic acid belongs to the group of B vitamins and is important during the process of growth and division, which is manifested in pregnancy where its deficiency in pregnant women who belong to the risk group can occur spina bifida, or neural tube defect of the newborn. It is also important for the maintenance of erythropoiesis, and if serum levels falls below 2 ng/mL, megaloblastic anemia occurs. Folic acid deficiency is common in people with inflammatory bowel disease. Mesalazine belongs to a group of anti-inflammatory drugs - aminosalicylates. It was developed after the realization that it is the active part of the drug sulfasalazine, which also contains the sulfapyridine part responsible for most side effects. It is effective in mild to moderate forms of ulcerative colitis, and is also used in Crohn's disease, although its effectiveness in this indication is questionable. In this work, the selectivity of the electroanalytical method for simultaneous determination of folic acid and mesalazine in Britton-Robinson buffer pH 6 was investigated, using a glassy carbon electrode modified by applying a 5 μL suspension of carbon nanotubes with bounded amino groups in 0,5% chitosan solution in 1% acetic acid at a frequency of 100 Hz, a pulse amplitude of 40 mV and a potential increase of 10 mV. Square-wave voltammograms of a mixture of mesalazine and folic acid do not show a change in the peak currents of oxidation of mesalazine and folic acid after the addition of inorganic salts (NaCl, KNO3, CaCl2 and NaHCO3) and glucose. Interference with mesalazine is shown by ascorbic acid and uric acid. However, interferences were observed for concentrations that were higher than those endogenously expected in the biological sample. The effect of other aminosalicylate drugs and mesalazine metabolites on the oxidation peaks of folic acid and mesalazine was investigated. Sulfasalazine and balsalazide interfere with both oxidation peaks, while olsalazine interferes only with the folic acid current peak, so it can be concluded that the newly developed sensor would be suitable for developing a method for the simultaneous determination of mesalazine and olsalazine. The mesalazine metabolite does not show interferences in the concentrations expected in plasma after biotransformation, so it would not affect the determination of mesalazine and folic acid by this method in the biological sample. Selectivity of the method was finally tested on samples of folic acid and mesalazine tablets where no interference of excipients with oxidation peaks nor the presence of new oxidation peaks derived from excipients was found.