The kinetics of isothermal pyrolysis of lignins occurring in hardwood found in the north of Argentina was studied using thermogravimetric techniques (TG/TGA). From the thermograms, which showed a similar behaviour in all the studied species, were determined the kinetic parameters for the isothermalpyrolysis in the temperature range between 226 and 435 degrees C using the Avrami-Erofeev equation for solid state reactions and the Arrhenius equation. The solid residues obtained at each temperature were analysed by infrared spectrophotometry in search for the modifications taking place in the different functional groups asa result of the temperature effect.