The use of acetone seeding as a fluorescence marker for mixture fraction in a laminar methane/air diffusion dame has been investigated by Laser Induced Fluorescence (LIF). The 308 nm radiation from a XeCl excimer laser excited both the acetone and hydroxyl radical first electronic transitions in a dame fueled by methane doped with 8.5% acetone. Flame structure, as indicated by hydroxyl radical profiles, is not appreciably different in the seeded flame compared with the unseeded flame. The raw fluorescence images are corrected for collisional quenching to produce acetone concentration profiles. A satisfactory correlation between acetone mass fraction and mixture fraction, xi, for the more fuel-rich regions of the dame is found. Acetone cannot be used in leaner regions because of thermal decomposition and slower diffusion for the dopant, with the former effect being more significant.