Experimental study is conducted to clarify the effects of hydrogen addition to biogas and hydrogen fraction in the biogas-H-2 mixture on the stability, thermal and emission characteristics of biogas-H-2-air premixed flames using a 9 mm-ID-tube Bunsen burner. Variation in biogas composition is allowed to range from BG(60) (60%CH4-40%CO2), down to BG(50) (50%CH4-50%CO2) and to BG(40) (40%CH4-60%CO2). For each biogas, the fraction of hydrogen in the biogas-H-2 mixture is varied from 10% to 50%. The results show that upon hydrogen addition and increasing hydrogen fraction in the fuel mixture, there are corresponding changes in flame stability, laminar burning velocity, flame tip temperature and CO pollutant emission. Under the tested conditions of 400 <= Re <= 800 and 0.8 <= Phi <= 1.2, the otherwise incombustible biogas-air mixture becomes flammable upon hydrogen addition and the threshold value of hydrogen fraction for occurrence of stable flames increases with the increase in the CO2 concentration of biogas. Overall, the flame stability is best at rich equivalence ratio of Phi = 1.2. The laminar burning velocity of the biogas-H-2-air mixture is found to be higher than pure biogas due to addition of hydrogen, and it monotonically increases at higher hydrogen fraction. Further, a shift in equivalence ratio for peak laminar burning velocity from stoichiometric Phi = 1.0 to Phi = 1.2 is observed. The results also show a monotonic increase in flame temperature and a monotonic decrease in CO emission at higher hydrogen fraction. The more efficient oxidation of CO into CO2 reveals that more complete combustion is induced by hydrogen addition. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.