A twin-plane electrical capacitance tomography (ECT) system has been employed to monitor the flow rate of fine coal transported by air in a 36.8 mm diameter pipe at mass fluxes of 1680 kg/m(2) s. The mean mass flow rate was obtained to be within < 1% of the value determined from load cells. The ECT output indicated that there were two types of systematic fluctuations in the time series of mass flow rate and concentration. The frequencies of these have been obtained from power spectral density analyses. For the higher frequency fluctuations, the Strouhal number (fD(t)/u(gs)) was found to depend on the square root of the solids/gas momentum flux ratio. An examination of the cross-sectional distribution of solids showed that the coal was concentrated around the wall of the pipe and that the concentration fluctuated as noted above. (c) 2008 Elsevier Ltd. All rights reserved.