Abstarct: Abstract In the current research work, a pencil graphite electrode (PGE) was modified using polyaniline/poly 2-methoxyaniline 5-sulfonic acid (polyaniline/PMAS) through electrochemical methods. The electrode was then employed for the determination of ammonium using cyclic voltammetry technique in aqueous solutions. In order to characterize the surface of the PGE modified with polyaniline/PMAS, chronoamperometry and impedance methods were utilized.To select a suitable working electrode, a comparative study was conducted between a glassy carbon electrode and the modified graphite pencil electrode. Considering advantages such as superior sensitivity, ease of use, and cost-effectiveness, the modified graphite pencil electrode was chosen over the glassy carbon electrode. The modified electrode with polyaniline/PMAS exhibited a better electrochemical response toward ammonium detection compared to the PGE, the PGE modified with polyaniline, the PGE modified with PMAS, and the PGE modified with polyaniline/PSS. Parameter optimization was carried out using experimental design methodology, including Plackett-Burman screening and Box-Behnken optimization.Under optimized conditions, the electrode demonstrated a good linear response in the range of 0.5-16.0 millimolar with a detection limit of 0.09 millimolar. Furthermore, the desired electrode exhibited good reproducibility. The interference effect of biological compounds and ions on the analyte measurement was investigated, and it was observed that most of the ions and compounds did not interfere with the analyte determination, indicating the electrode selectivity.The use of the modified electrode with polyaniline/PMAS was successfully applied for the determination of ammonium in samples of the urban water and industrial wastewater of Shahroud Industrial City.