We present experimental observations of the density field and reaction structure of methane and natural gas detonation waves propagating in a narrow channel. Simultaneous time-resolved schlieren and CH chemiluminescence images are used to describe the structure of the unstable front. Nitrogen dilution concentration is varied, and the effect of increasing dilution is to increase the instability level and cell size and decrease the chemiluminescence intensity. Comparison is made between methane- and natural gas-fueled detonations. The effect of the higher hydrocarbons present in natural gas, primarily ethane, is to increase the fine-scale structure of the detonation front and create a more continuous reaction front. Utilizing the simultaneous images, observations are made about the formation and dissipation of the material separated across the shear layer behind the front.