The characteristics (flame length, pollutant emission, radiative heat loss fraction, and volumetric soot concentration) of hydrogen-hydrocarbon composite fuel turbulent jet diffusion flames are presented. A correlation of flame length with hydrogen concentration in the fuel mixture is shown. The reactivity of fuel mixture increases with the increase of hydrogen concentration, which ultimately shortens the combustion time, and thereby reduces the overall flame length. Convective time scale decreases with the increase of hydrogen content in the mixture. The measured and predicted flame lengths show a similar trend; however, the predicted values are 1.4 times higher than the measured values. Axial soot concentration decreases, the CO emission index decreases, but, NO and NOx emission indices increase at higher hydrogen concentrations in the mixture. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.