This paper presents experimental measurements of the critical velocity gradient and flashback behavior of H(2)-CO and H(2)-CH(4) mixtures. Effects of H(2) concentration, external excitation, and swirl on the flashback behavior for flames of these fuel mixtures are discussed. For H2 concentration burner and scaling studies the critical velocity gradient (g(F)), defined as the ratio of the square of the laminar burning velocity to the thermal diffusivity of the mixture (g(F) = S(L)(2)/alpha), was used to quantify the flashback propensity of the flames. The critical velocity gradient of both H(2)-CH(4) and H(2)-CO flames changed nonlinearly with the increase in H2 contents in the mixture. The critical velocity gradient (g(F)) of 5-95% and 15-85% H(2)-CO mixtures somewhat agreed with the scaling relation (g(F) = c S(L)(2)/alpha) and yielded an average c value of 0.04. Similarly, values of a 25% H(2)-75% CH(4) for different burner diameters were also fitted using the scaling relation yielding an average c value of 0.044. The gF values of 25-75% H(2)-CO mixture showed non-linear variation with the S(L)(2)/a ratio (especially for S(L)(2)/alpha > 19; 000 s(-1)), and at a lower S(L)(2)/alpha ratios burner diameter had small effect on critical velocity gradient measurements. The opposite trend was observed for a 25-75% H(2)-CH(4) mixture showing non-linear variation at a lower S(2)(L)/alpha ratios (for S(L)(2)/alpha > 5600 s(-1)) and having less effect at higher S(L)(2)/alpha ratios. It was also determined that the effect of external excitation on the flashback propensity of H(2)-CO flames with more than 5% H(2) was not significant. Flashback through two mechanisms and their dependence on combustor parameters were also identified for swirl stabilized H(2)-CO flames. (C) 2010 Elsevier Ltd. All rights reserved.