An investigation is reported into the deposition of pulverized coal and cement within a 53 mm bore mild steel horizontal pipeline under dilute-phase conveying conditions. Deposition of this kind may have significant effects on the response of flow measuring instruments and also on the conveying efficiency. The degree of deposition was continuously monitored during the conveying process so that time variations were recorded. Data were simultaneously derived from both radiometric and electrodynamic sensors, thus allowing on-line discrimination between moving and stationary solids within the pipeline. Measurements were made over a range of particle sizes, solids velocities and solids concentrations. Deposition is found to have a complex dependence on these variables.