This paper reports an experimental study of the NO-char reaction using two types of biomass-wheat straw and eucalyptus wood. The experiments were performed in a fixed bed reactor at 1023-1173 K with NO inlet concentrations ranging from 50 to 1500 ppmv. The char was produced in-situ at the reaction temperature, and a special interest of the study was to measure the initial rate constant before significant char thermal deactivation had occurred. Separate experiments were carried out to investigate when the influence of volatiles on NO reduction could be neglected and the observed rate constant was only due to the heterogeneous NO-char interaction. The kinetic expressions for the initial rate obtained under these dynamic conditions are the following: -r(NO) = 3.26 x 10(4) x exp(-11092/T) x C-NO(0.45) mol/(kg C in char(.)s) for wheat straw, and -r(NO) = 3.55 x 10(3) x exp(- 9305.5/T) x C-NO(0.3) mol/(kg C in char(.)s) for eucalyptus wood. For comparison also the reaction rate constant at pseudo steady-state conditions was determined and found to be about a factor of 3 lower than the initial rate. Both chars have reactivities from 1 to 2 orders of magnitude higher than typically reported for coal on a mass basis. This is attributed partly to a generally higher reactivity of biomass compared to coal and partly to significant thermal annealing of the coal chars. The experiments confirm that thermal deactivation is important for the NO-char reaction, but adsorption of NO on the fresh char also contributes to the high initial rate.