An experimental technique has been developed to measure both the residence time distribution (RTD) and the mass rate of solid particles for a down flow of solid-gas mixture in a vertical reactor. This technique is based on the emission by a Light source and the reception by a photoelectric cell of a light beam crossing the bottom part of the reactor through which flows a given amount of solid particles, initially introduced as an impulse at the top of the reactor. Effects of the gas flow rate and the particles diameter have been studied. It has been shown that an increase of the gas flow rate or the particle diameter leads to a decrease of both the peak emerging time t(p) and the width of the peak (t(p) - t(1)). Moreover, this technique has proven to be appropriate for the measure of the particle mass rate.