Thymol and carvacrol (isopropylmethylphenols) are phenolic monoterpenoids with antibacterial, antifungal, insecticidal, and antioxidant properties. Thyme, oregano, and other culinary and medicinal herbs are their natural sources. The dose-dependent antioxidant effect of isopropylmethylphenols requires their control in real samples. The presence of phenolic hydroxyl group in the thymol and carvacrol structures makes them electroactive at the electrode surface allowing application of voltammetry for their quantification. Glassy carbon electrode modified with multi-walled carbon nanotubes and electropolymerized thymolphthalein has been developed for this purpose. Conditions of thymolphthalein electropolymerization (monomer concentration, number of cycles, parameters of electrolysis) providing the best response to thymol have been found. Scanning electron microscopy and electrochemical methods confirm the effectivity of the sensor developed. According to voltammetric data, both thymol and carvacrol are irreversibly oxidized at almost the same potential with the participation of two electrons and two protons forming thymoquinone. In differential pulse mode, the sensor gives a linear response in the ranges of 0.050-25 and 25-100 µM for thymol and 0.10-10 and 10-100 µM for carvacrol with detection limits of 0.037 and 0.063 µM, respectively that is significantly improved compared to reported earlier. Sensor developed is selective to isopropylmethylphenols in the presence of typical interferences (inorganic ions, saccharides, ascorbic acid) and other phenolics (caffeic, chlorogenic, gallic and rosmarinic acids, quercetin, and rutin). Sensor has been applied for the evaluation of total isopropylmethylphenols in oregano and thyme spices using single sonication assisted extraction with methanol. Voltammetric sensor data agree well with the independent spectrophotometric quantification.