New, technically sophisticated measurement techniques must not be used blindly. In order to ensure accurate measurements, the limits of such devices must be well known. In this study a single-fiber optic reflection probe was thoroughly tested in two-phase gas-solid flows for the measurement of particle velocity and concentration. A novel, dynamic calibration procedure for particle concentration measurements has been proposed and tested in the downflow reactor section of a pilot-scale circulating fluidized bed. The results show that, as a particle concentration measuring device, the probe is very sensitive to electrostatic effects in the flow medium. The velocity measurements are relatively unaffected and were shown to be reproducible with errors of between 10 and 15% for particle velocities of up to 8 m/s.