This paper presents the results of experimental studies conducted on premixed CNG/H-2/CO/CO2 blending synthetic gas flames in a laboratory scale combustor, with particular emphasis on effects of swirl number (from 0.2 to 1.6, at intervals of 0.2) and equivalence ratio (0.6, 0.8 and 1.0) on combustion and emission characteristics of such mixtures. Furthermore, effects of swirl number on lean blowout and flashback limits of studied mixtures were also investigated. Irrespective of the swirl number and equivalence ratio, H-2/CO ratios of tested gas mixtures were kept constant (1.0), and mixtures of H-2/CO/CNG/CO2 which were diluted with different amount of CO2 (0-20%, at intervals of 5%) were derived. Considering broadest operating range, mixture of 20%CNG/30%H-2/30%CO/20%CO2 was determined as base fuel to be tested. Combustion and emission behavior of tested gas mixtures were assessed by examining axial and radial temperature, NOx, CO and CO2 distributions. Besides, swirl number and equivalence ratio effects on flame structure were evaluated by investigating instantaneous flame images (with the same focal length and exposure time). Results of this study showed that effects of swirl number on flame characteristics (temperature and pollutant emission distributions) is not monotonous, and equivalence ratio dependent flame behavior alters differently at different swirl numbers. (C) 2019 Elsevier Ltd. All rights reserved.