A new probe based on the measurement of electrical impedance has been developed to measure solid/liquid hold-up in stirred tank reactors working with high solid concentration. The main advantage of the probe is that it is fairly non-intrusive and can be used for both laboratory-scale and industrial stirred tank reactors. The relationship between impedance and liquid hold-up closely follows the predictions estimated with the Bruggeman's model. The solid concentration was measured in a 0.5-m diameter vessel mechanically stirred with a Lightnin A310 impeller, set at an off-bottom clearance C = 0.25T Three sizes of Lightnin A310 impeller were used corresponding to impeller to tank diameter ratios of D/T = 0.346, 0.386 and 0.5. The liquid phase was tap water, whereas two different sets of spherical glass beads (of density p, = 2500 kg/m(3)) with narrow size distributions and mean diameters d(p) = 90 and 600 mum, respectively, were used as solid phase. Solid loads up to 40% by weight were investigated. In addition to solid concentration, power consumption was measured by means of strain gauge technique, and interface height was determined. Preliminary results show that dimensionless axial dispersion coefficient, D-es/ND2, decreases with increasing solid load.