A mass stream of a dusty solid fuel is conveyed pneumatically through a supply pipe leading from a dosing container to a gasification reactor. A gas admixing device is connected in series with the supply pipe near the outlet of the container for injecting an auxiliary gas into the mass stream. The parameters of the mass stream are measured before and after the mixing device and applied to a computer which determines the amount of the injected gas and the value of the mass stream. In this manner, the arrangement for measuring the gas stream can operate continuously and independently from the technology of feeding the dust to the supply pipe and provides quantitatively correct measuring values at all dust concentrations and system pressures. The method eliminates complicated velocity measurements in two-phase dust streams and does not necessitate any measuring probes which project into the conveying channel and obstruct the flow.