The possible burning structures existing in two co-flowing combustible mixtures with different compositions, and their implications to the field of turbulent combustion are examined in this study. A coaxial burner with a quartz plate was used to experimentally investigate the flames of methane/air and propane/air mixtures propagating in a coaxial flow impinging onto a stagnation surface. The possible burning structures were observed to be: (1) a single-flame (a lean or rich premixed flame); (2) a double-flame (two lean or rich premixed flames, or a rich premixed flame and a diffusion flame); and (3) a triple-flame (a rich premixed flame, a diffusion flame and a lean premixed flame). An inner (or outer) mixture, far beyond the flammability limit, can still burn if a stronger outer (or inner) flame supports it. The extinction limit of the top part of the inner hat-shaped premixed flame is nearly independent of the burning intensity of the outer flame. It was found that the inner flame has a wider flammable region than the outer flame, and that the latter has a narrower flashback region than the former. Both propane and methane flames may exhibit flame-front instability, although the former displays much more clearly than the latter. Cellular and polyhedral instabilities can exist individually or appear simultaneously in the inner flame. However, only polyhedral (stripped-pattern) instability was observed in the outer flame. Finally, the experiments were analyzed theoretically using a simple geometrical model incorporated with the numerical simulations. The predicted shapes and locations of the flames are in good agreement with the experimental observations qualitatively. (C) 2003 The Combustion Institute. All rights reserved.