Simultaneous Raman-Rayleigh-fluoresence measurements have been made in turbulent jet diffusion flames of CO/CH 4/N 2 and CO/H 2/N 2 fuel mixtures. Species mass fractions of CO, CO2, CH4, H2, H20, 02 and N 2 are obtained from the Raman data, density and temperature from the Rayleigh data, and the OH radical from the laser-induced fl¢orescence measurements. The fairly high N 2 dilution, by suppressing chemiluminescence interference, results in improved quality Raman data. The extinction behavior of all the flames tested is qualitatively similar to that of diluted flames of CH4: as the jet velocity is increased, extinguished parcels appear more and more frequently in the region of highest mixing rates, until finally reaction downstream of this region is no longer sustained. For the CO/CH 4/N 2 flames extinction occurs throughout the reactive zone, whilst for the CO/H 2/N 2 flame, extinction is seen mainly on the lean side of stoichiometric composition. One effect of CH 4 admixture is to narrow the reaction zone on the rich side through radical depletion by the fuel. As expected, it is found that the CO-CO 2 conversion is far from being in partial equilibrium in these flames.