Pulse cleaned rigid ceramic filters are widely used for particle removal from gas streams at elevated temperatures; when impregnated with SCR catalyst material, such filters can be used to filter particles and remove NOx simultaneously. The effect of cyclical variations of filtration velocity on catalytic conversion was modelled for different scenarios and measured. A model was developed which accounts for changes in gas contact time resulting from periodic filter cake build-up and detachment when filter elements are regenerated in clusters or incompletely. It then estimates the NOx conversion using reaction-kinetic data obtained elsewhere. It predicts that variations in local face velocity generally reduce the overall NO conversion. Incomplete filter regeneration with locally fixed patchy cleaning can have a significant influence whereas the influence of locally coincidental patchy cleaning and cluster-wise regeneration is low (reduced NO conversion on the order of 3% points). The effect of incomplete cake removal on NO conversion was measured at 300 degrees C using filter discs and for biomass combustion relevant ash/sorbent mix. The results generally agree with the model calculations, confirming the small influence of random patchy cleaning. The effect of particle penetration into the filter as well as mechanical stress due to back-pulsing on the NO conversion was also found to have a small influence. High NO conversion could be maintained after 24 h of operation at technically relevant face velocity. (C) 2007 Elsevier B.V. All rights reserved.