The fluctuating component of the solid fraction is studied for liquid-solid mixtures using an impedance volume-fraction meter (IVFM). The experiments involve a vertical gravity driven flow using glass particles in water, and a liquid fluidized bed with either glass, steel or nylon particles. The signals of the IVFMs for different flow conditions are analyzed in time and frequency domains. Results show that the magnitude of the root-mean-square (RMS) solid fraction fluctuation increases from low concentrations (<10% solid fraction), reaches a local maximum at approximately 30% solid fraction, decreases again and reaches a maximum at concentrations around 45%. For higher concentrations, the RMS fluctuations decreases until the packed state. For the same solid fraction the magnitude of the fluctuation increases for larger diameter particles. Two main types of fluctuations were identified: large-amplitude low-frequency fluctuations and small-amplitude high-frequency fluctuations. The low-frequency fluctuations were dominant at high concentrations (>30%), while the high-frequency fluctuations became more prominent for concentrations below 30%. When the low frequency fluctuations were filtered out and the RMS fluctuation was re-calculated the scaling appear to change: the filtered RMS fluctuations were larger for experiments with higher particle inertia. A comparison between the filtered RMS solid fluctuations and an exiting model showed limited agreement. (C) 2000 Elsevier Science Ltd. All rights reserved.