Stable carbon isotope analysis, a unique analytical technique, has been utilized for distinguishing and quantifying the individual contributions of coal and biomass feedstocks in the generation of carbon containing gases during the gasification of their blends. For this purpose, corn stover (CS) and switchgrass (SG) were individually blended up to 30% by weight with two different Montana coals, namely, DECS-38 sub-bituminous coal (SB) and DECS-25 lignite coal (LG) and gasified at atmospheric pressure with varying O-2/steam ratios. Gasifying at a constant O-2/steam ratio while increasing percentage of biomass in the feedstock resulted in an increase of delta C-13 MO values for the carbon containing product gases. An increase in the concentration of oxygen in the feed stream at a constant biomass percentage leads to the depletion of (CO)-C-13 and enrichment of (CO2)-C-13 in the blends with corn stover while an enrichment of both (CO)-C-13 and (CO2)-C-13 was observed in blends with switchgrass. Based on isotope mass balance considerations, the contribution of corn stover was in the range of 1% - 40% for CO generation and 6% - 69% for CO2 generation. Switchgrass, on the other hand, contributed up to 53% for CO generation and only 46% for CO2 generation. (C) 2014 Elsevier B.V. All rights reserved.