Measurement of solids concentration in aqueous slurries is of practical importance in many industries. In this research work, a guided microwave total transmission technique was developed to measure the solids concentration of slurries. A novel approach was used to calculate the complex permittivity of slurries and to minimize the measurement errors. Experimental results were obtained for various differently concentrated kaolin-in-water slurries. Quantitative relations between the complex permittivity and volumetric fraction of the suspended solids in slurries were obtained. These data could be used to model the dielectric behavior of slurries in microwave frequency region and to provide definitive information about the solids concentration in slurries. The effect of ions on the complex permittivity of slurries at microwave frequency was also examined experimentally.