The initial stage of the polymerization of coniferyl alcohol catalyzed by a peroxydase was studied at the air-water interface. The properties of the monolayers were investigated at constant area and at constant surface pressure by surface pressure, surface potential and ellipticity measurements. On the basis of the results obtained at constant surface area, it is proposed that the formation of a 2D layer occurs up to the inflection point of the surface pressure-area isotherm, and that for larger surface pressures a 3D structure is formed during the polymerization. If the barostat is set at the inflection point (the surface pressure is constant while the surface area increases), the 2D organization of the monolayer is retained. A kinetic model describing the adsorption of the reaction products in a 2D layer is proposed. The kinetic constants were determined from the surface potential, ellipsometry, surface pressure and surface area data.