Bioadditives are often used around the world as a part of the daily human diet. Contrary to pharmaceuticals, bioadditives are not subject to rigorous quality control, and their full chemical composition is usually unknown. Therefore, the determination of active components in bioadditives is of high importance and can be achieved using voltammetry. Novel voltammetric sensors were developed for the quantification of L-tyrosine and diosmin in bioadditives. A glassy carbon electrode (GCE) modified with SnO₂ nanoparticles dispersed in sodium dodecyl sulfate and electropolymerized Eriochrome Black T allowed for the determination of L-tyrosine. The effect of its surfactant nature was tested. A GCE covered layer-by-layer with carboxylated multi-walled carbon nanotubes and polydopamine responded to diosmin. The conditions of electropolymerization were optimized using a target analyte voltammetric response. The electrodes' surface morphology and electron transfer properties were estimated by means of scanning electron microscopy and electrochemical methods. The increases in the electroactive surface area and electron transfer rate were confirmed. The electrooxidation parameters of L-tyrosine and diosmin were found. The electrodes were used as voltammetric sensors in Britton–Robinson buffer with a pH of 2.0 in differential pulse mode. The linear dynamic ranges of 0.75–100 μM for L-tyrosine and 0.75–25 and 25–100 μM for diosmin were achieved, with detection limits of 0.66 and 0.25 μM, respectively. The selectivity of the sensors’ response to target analytes in the presence of typical co-existing compounds was proven. The practical applicability of the developed sensors was shown on real samples. A comparison to standard high-performance liquid chromatography confirmed similar levels of precision of the methods.