A comparative study of the flame structure and characteristics of diffusion flames of the mixture of hydrogen-hydrocarbon (natural gas and propane) hybrid fuel in a slow co-flowing stream of air is presented. The volumetric content of natural gas and propane in the mixture was varied from 0-35%. The burner exit Reynolds number was varied from 150-3000. Measurements include flame length, radiative fraction of heat release, pollutant emission indices and in-flame profiles of composition and temperature. Results indicate: the increase of natural gas or propane in the hydrogen-hydrocarbon mixture increases the flame luminosity and flame length, increases the radiative heat loss fraction and soot concentration, decreases both NO and NOx emission indices, increases the CO emission index, and decreases the peak temperature in the near burner, mid flame and far-burner regions. The changes are quantified in the paper. (C) 2000 International Association for Hydrogen Energy.