Abstarct: In the present study, the modification of electrode surface with poly(2-methoxy aniline-5-sulfonic acid) (PMAS) as an electroactive conductive polymer was investigated. The electrode surface was modified with PMAS via different methods of cyclic voltammetry, constant potential, dipping and drop casting and electrochemical polymerisation of polyaniline was then carried out at the surface of modified electrode (ME). Effect of different variables on the modifying laxyer obtained using drop casting method (as the optimum modified electrode) was also investigated. The parameters included solution concentration, volume and pretreatment of the ME. The polymer synthesized at the ME was characterised by CV and UV-vis spectrophotometry techniques. The results revealed that PMAS was present and a stable and electroactive PAn film was formed. Electrochemical experiments showed that even at very low concentrations of aniline (4.0 mM), an adherent, uniform and stable polyaniline film was deposited on the electrode surface. Also, the electrochemical activity of the resulting polyaniline was investigated in phosphate buffer solution in a wide pH range. The results obtained indicated that polymer was electroactive at neutral pH. Polyaniline film on the ME as an electron transfer mediator exhibited electrocatalytic activity for the oxidation of ascorbic acid.