Blending hydrogen into hydrocarbon fuels can reduce the carbon-intensity of the fuel and extend the lean flammability limit. However, limited information is available of the global performance of attached, non-piloted hydrogen-hydrocarbon jet flames under well-defined boundary conditions. Three groups of blended fuels were investigated in the current study: Natural gas +H-2 (with H-2 volume fraction varying from 18.6% to 100%), C2H4 + H-2 (with H-2 volume fraction varying from 0% to 100%), and 40% C2H4 + 41% H-2 + 19% N-2. Measurements were performed of flame dimensions, radiant fraction, and emission indices of NOx, and CO. For flames with constant exit strain rate, the increase of hydrogen volume fraction was found to decrease the radiant fraction, decrease the global residence time and increase the NOx emission index. For flames of the same fuel composition, a higher strain rate results in a lower radiant fraction. The NOx production rate scales with the reciprocal of non-adiabatic flame temperature, consistent with the thermal NOx mechanism. The CO/CO2 ratio is determined by the competing influences of flame residence time, carbon input rate and mixing rate of the fuel and air. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.