Fuel blending represents a promising approach for reducing both NOx and particulate emissions from flames. This paper reports a fundamental investigation on the effects of blending hydrogen with different fuels (methane and n-heptane) on the structure and emission characteristics of counterflow nonpremixed and partially premixed flames (PPF). The emission behavior is characterized in terms of the concentrations and emission indices of various pollutant species as a function of hydrogen content in the blend. Results indicate that hydrogen blending has a much more favorable effect on emissions in heptane flames than in methane flames. With hydrogen addition in methane/hydrogen blends, the emission index of C2H2 (which is an important soot precursor) is reduced. CO remains unchanged. and NO increases slightly. In heptane/hydrogen blends, however, emission indices of all three species (NO, C2H2, and CO) decrease significantly with hydrogen addition. The behavior is attributed to two factors. First. for the same strain rate, the addition of hydrogen decreases carbon content in the fuel. which reduces the amount of CO and C2H2 formed. Because C2H2, is the major source of CH that leads to the formation of prompt NO, and because prompt NO is the dominant contributor to total NO in heptane flames. hydrogen addition leads to a dramatic decrease in NO emission in these flames. Second. the addition of hydrogen changes combustion chemistry due to the higher reactivity of H, and the higher concentrations of H and OH radicals, which increase the CO and C,H, oxidation rates. In addition, this enhances the C, path for methane oxidation and decomposition of n-heptane to C-3-C-6 species and, thus, decreases the formation of C,H,. The net result is a decrease in the emission of all three pollutant species, although the effect is significantly higher for n-heptane flames. For PPFs, the addition of hydrogen increases the physical separation between the two reaction zones-namely, the rich premixed and the nonpremixed zones (i.e., the flames become broader). The emission of NO, C2H2, and CO species is also affected favorably due to hydrogen addition, but the effect is less significant for PPFs compared to that for nonpremixed flames.