The original contribution of this work to the knowledge lies in experimental determining how the laminar burning velocity (Su0) and Markstein length (Lb) of synthetic biogas and syngas/CH4 mixtures, with close to real gas composition, are affected by simultaneous variation of composition, equivalence ratio and pressure. This objective is accomplished by conducting a multifactorial analysis, based on the design of experiments (DoE) statistical technique, on the measured Su0 and Lb of the gases mixtures. The experimental parameters were obtained using a cylindrical constant-volume vessel with optical access that was designed, manufactured and validated. The investigated variables for both biogas and syngas mixtures are the initial pressure (P0) and the ratio CO2/N2 ranging from 1 to 5 bars and 0.33 to 5, respectively. For biogas, the other investigated variables are the fuel/oxidizer equivalence ratio ( and the volume CH4 % ranging from 0.7 to 1.1 and 45 to 65 %, respectively. For syngas, they are and the degree of dilution (DoD) ranging from 0.5 to 1.1 and 20 to 60 %, respectively, while CH4/H2 and CO/H2 range from 0.33 to 3. The experimental data have been analyzed using the statistical technique of multiple linear regression model. The significance of the results for Su0 is higher than 95 % for most of the variables under analysis, thus providing reliable prediction profiles whereas for Lb the significance of the results is generally lower. P0 and are the variables more strongly affecting Su0 of biogas. The variable CO2/N2 is negligible compared to the others. is the most important variable affecting Lb followed by P0 and CO2/N2. The variable CH4 % is negligible compared to the others. The most important variables affecting Su0 of syngas/CH4 mixtures are and DoD, followed by CH4/H2, P0, and CO2/N2. The CO/H2 variable is negligible compared to the others. The most important variable affecting Lb is , followed by CH4/H2, CO/H2, P0 and CO2/N2. The importance of the degree of dilution is negligible compared to the others. The negligible importance of CO/H2 for Su0 and DoD for Lb is in contrast with literature and is explained by the low significance of those results, i.e., the model is not able to determine a clear relationship with Su0 and Lb, respectively. The conditions under which buoyancy and cellular instability become relevant for biogas mixtures are low equivalence ratio ( = 0.7) and high pressure for buoyancy and = 0.9 and high P0 for the onset of cellularity. For syngas/CH4 they are low equivalence ratio ( = 0.5), high DoD and high P0 for buoyancy and Lewis number lower than unity and high pressure for the onset of cellularity.