The experimental studies and mathematical modeling are presented for the residence time distribution of fine solids moving in a co-current downflow fixed-bed reactor. Experiments were carried at ambient conditions using a glass column of 30mm i.d. packed by spheres of 4-6 mm diameter. Fine alumina of 50-250 mum was used as a moving solids. Solids flow rate was varied from 0.1 to 0.5 g cm(-2) s(-1). The new flight-time technique is applied to measure residence time of fine particles in a packed bed. The influence of particle size and flow rate on the particles residence time is experimentally investigated. New three-dimensional model of moving of fine particles in a packed bed is developed. The model is based on the statistical calculations of an individual particle path in a packed column. Model validation was checked by comparing experimental values of particles residence time distribution with calculated one. Calculated values of particles residence time were found to be in a good agreement with experimental data at the solids flow rate below 0.1 g cm(-2) s(-1)