This paper describes gas-phase residence time distribution (RTD) measurements using a novel method for fast concentration measurements of binary gas mixtures that is based on differences in the ultrasonic velocities of the mixture's components. It is shown that systems with holding times as low as 0.1 s can be interpreted, although an increase of the sampling frequency would be desirable. The technique was validated using single-phase gas reactors (tubular reactors and an intensely stirred reactor). The measured concentration, average residence time, and degree of mixing in the single-phase reactors were in good agreement with the experimental conditions and results from the literature. The method was applied to study the hydrodynamics of a novel gas-solids multistage circulating fluidized-bed reactor for biomass gasification, using a prototype measurement cell to evaluate the gas-phase RTD. From the measured RTD curves of this reactor, which consists of five fluid-bed stages, it can be concluded that there are overall no dead zones/ bypass streams with low exchange rates to the main flow field. The gas-phase hydrodynamics can be described by a series of six CISTRs compared to the five actual fluid-bed stages of the base-case design. The employed ultrasonic detection technique has several advantages. The composition of binary gas mixtures can be measured at a high sampling frequency in the presence of solids, and moreover, the average local. solids fraction can be determined simultaneously. The method can be completely noninvasive, and measurements through reactor walls and even in industrial installations are expected to be possible.