This study considered an infrared extinction technique for the measurement of hydrocarbon vapor in two-phase flows. The focus of the study was the assessment of the abilities and limitations of the technique in order to establish its viability as a tool in the laboratory. The approach taken was to provide a controlled environment by simulating a fuel spray with styrene beads and methane and confining that flow to a test section. This allowed properties such as the vapor concentration and the particle optical thickness to be controlled directly. The vapor flow was fixed because there was no droplet evaporation. The size distribution of the particles could be determined a priori. Vapor mole fractions were measured in the single phase for concentrations from 0.15% to near 20%. These limits were dependent on several experimental parameters, yet they indicated the wide range of vapor mole fractions that could be measured with this method. Vapor mole fractions were measured in a two-phase environment for a variety of conditions. These results indicated that vapor mole fractions could be measured in a two-phase environment with an error of less than 10% for most cases. A sensitivity analysis was performed by examining sources of error and numerically varying measured parameters to see which parameters were critical to the final result. The analysis indicated that the technique in general was quite insensitive to the physical characteristics of the particles, provided that they were much larger than the wavelength of the incident light and that careful consideration of the measured collection angle was made. Thus, properties such as the size distribution of the particles and their refractive index may have surprisingly little, if any, impact on the determination of the vapor mole fraction.