Catalytic oxidation of NO (380 ppmv) to NO2 over activated carbon fibres of moderate surface area (similar to 800 m(2) g(-1)) at room temperature was carried out, to develop oxidative removal of NO from flue gas. The heat treatment of pitch-based activated carbon fibres of moderate surface area markedly increased the conversion of NO to 87% in dry air, 62% in air of 80% relative humidity and 24% in wet air (100% r.h.) at 25 degrees C and a ratio of fibre mass to gas flow rate of 1.0 x 10(-2) g min mL(-1). The strong inhibiting effect of humidity on the activity of the as-received fibres was moderated by heat treatment of the fibres at 850 degrees C. A lower concentration of 10 ppmv NO markedly reduced the oxidation, the conversions being 24 and 5% in dry and wet (100% r.h.) air respectively. The catalytic activities of other pitch-based fibres of different surface area were also much inferior in moist air above 60% r.h.