In this study; effects of fuel composition, equivalence ratio and swirl number on flame behavior (dynamic and static) of premixed 100% CNG, CNG-H-2, CNG-H-2-CO and CNG-H-2-CO2-CO mixtures under externally altered acoustic conditions were experimentally investigated in a laboratory scale swirl stabilized combustor. During experiments, the amount of CNG in all gas mixtures tested was set as 20% and 40% by volume except for the CNG-H-2 mixture. Moreover, H-2/CO ratios of CNG-H-2-CO mixtures were arranged to be able to provide low, medium or high heating value. To better represent synthetic gas, CNG-H-2-CO mixture was then diluted with the same amount of CO2, and mixtures of CNG-H-2-CO2-CO with varying H-2/CO ratios were achieved to evaluate effects of CO2 dilution (in addition to flame behavior) on flame characteristics (i.e. performance metrics such as flame temperature and emissions). Acoustic field of the combustor was altered via side mounted loudspeakers to trigger combustion instabilities, and under these circumstances, flame behavior was evaluated by examining instantaneous flame images, pressure and luminous intensity profiles. Data obtained from pressure transducers and photodiodes was also utilized to detect whether dynamic instabilities, i.e. thermoacoustic instabilities, excite static instabilities such as blowout, flashback, and flame liftoff etc. or not. Results of this study showed that fuel composition, equivalence ratio and swirl intensity are determinant parameters on flame behavior. Two main inferences of this study are: swirl number has a monotonic impact on flame stability characteristics (1); under externally excited acoustic conditions, CO2 presence in gas mixtures makes flames more resistant to dynamic instabilities (less axial and radial oscillations) and blowout but it increases flashback tendency (2).