Deactivation of SCR catalysts applied in bio fuel plants is a major problem due to higher amounts of potassium in bio fuel compared to other fuels. Regeneration of deactivated catalysts seems to be a promising way for minimising the total cost of bio fuel plants and great potential lies in finding regeneration processes that can be used commercially. The first applied regeneration method was washing of two different commercially aged catalysts in different aqueous solutions. The other method was washing with water followed by sulphation at different temperatures. Sulphation with SO2 resulted in higher activation without affecting the amount of potassium accumulated on the surface indicating the role of surface sulphate groups. Since potassium both decreases the catalytic activity for NO-reduction and retards the redox potential of the surface vanadium species during the sulphation procedure, it is necessary to wash the heavily deactivated catalyst before sulphation. Washing with water or diluted sulphuric acid could not restore the vanadium groups to the initial condition. Washing with 0.5 M H2SO4 was the most effective regeneration method and the bio-modified catalyst regained 92% of its initial activity, while the corresponding value for the conventional catalyst was 111%. (C) 2001 Elsevier Science B.V. All rights reserved.