Abstract: A method of measuring concentration of solid or liquid particulate matter in a gaseous carrier medium, particularly suitable for measuring concentration and flow rate of pulverized fuel pneumatically transported in a duct to power station boiler burners. The method includes generating a radio frequency electromagnetic wave within a duct, measuring wave parameters within a range of frequencies, including a cut-off frequency, determining the cut-off frequency from changes in wave parameters, calculating dielectric permittivity of a two-phase flow duct load, and ascribing to this value a corresponding concentration value. The measured parameter is the rate of change of phase shift against frequency at two arbitrary points on a wave transmission path, whereas the waveguide cut-off frequency value is determined at a point where the rate of change of phase shift against frequency reaches a maximum.