Polyaniline and its derivatives have gained a unique interest because of their chemical stability and stimulating physical properties which strongly depend on their preparation procedure. The determination of a monomer transport from the bulk of the solution to the electrode surface is one of the key factors to understand the initiation process of the polymerization. In this paper the results of the study on diffusion coefficient (D) for aniline (ANI), o-methoxyaniline (OMA) and o-ethoxyaniline (OEA) are described by means of the rotating disc electrode (RDE) technique. The calculated values of D are 2.4 x 10(-6) cm(2) s(-1), 3.9 x 10(-6) cm(2) s(-1) and 4.7 x 10(-6) cm(2) s(-1) for ANI, OMA and OEA, respectively. D value increases with the length of the alkoxy substituents, which indicates a faster molecular movement with the increase in molecular dimensions. The proposed explanation for this dependence is based on the real Stokes radius of a diffusing monomer, which depends on the number of water molecules forming a water shell around monomer molecule which decreases with the length of alkoxy substituents. The increasing diffusion coefficient values with the length of the alkoxy substituents correspond well with the increase in the determined rate constants, showing the diffusive control of the oxidation of the monomers to a radical cation under studied experimental conditions. (C) 2010 Elsevier B.V. All rights reserved.