A novel spectroscopic method was developed to measure quantitatively and simultaneously solids volume fraction (1-epsilon) and gaseous species composition (Y-i) in a gas/solid system. The method was comprised of an FT-IR coupled to a fiber-optic probe that could perform real-time and in situ measurements of absorbance. The effect of (1-epsilon) and Y-i on the absorbance spectra were additive and could be independently calibrated. Experiments were conducted with alkane/nitrogen mixtures and two types of particles: sand and FCC. Fuel mole fractions and (1-epsilon) were varied between 1.8-10.1 mol % and 0-0.45, respectively. The relative errors for Y-i time-averaged measurements were below 6% and the error increased significantly with decreasing beam intensity. A proof of concept for a novel application in fluidized beds was also completed: the fiber-optic probe was used to measure the molar fraction of a tracer gas inside the emulsion and bubble phases during gas tracer experiments. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 2850-2859, 2010