A system of Pt nanoparticles and poly(ortho-phenylenediamine) film electrochemically deposited onto a glassy carbon electrode (GCE/PoPD/Pt) was fabricated. Scanning electron microscopy, Fourier-transform infrared spectroscopy, and atomic force microscopy techniques were used to identify the surface characteristics of the composite electrode. The conductive polymers and Pt nanoparticles together resulted in a synergistic effect, and the new formed surface was highly active against polyphenolic structures. Rosmarinic acid (RA) and protocatechuic acid (PCA) are phenolic compounds found in plants, and they are used in many applications, particularly as pharmaceuticals. The GCE/PoPD/Pt was used for the simultaneous determination of RA and PCA in a pH 2.0 H2SO4 solution for the first time. The RA and PCA concentrations were determined using differential pulse voltammetry (DPV) and chronoamperometry. By the amperometry measurement, for RA and PCA, a linear relation was observed in the concentration ranges of 1-55 mu mol L-1 and 1-60 mu mol L-1, with detection limits of 0.5 mu mol L-1 and 0.6 mu mol L-1, respectively. In the simultaneous determination with DPV, the detection limits for both RA and PCA were calculated as 0.7 mu mol L-1. The GCE/PoPD/Pt was successfully used for the simultaneous determination of RA and PCA in a real sample, and its accuracy was verified by high-performance liquid chromatography studies.