The non-invasive measurement of mixing inside a stirred vessel in 3-D, using an 8 x 16 array of resistance tomography sensors, provides powerful opportunities for characterising and quantifying the process complexities [R. Mann, F.J. Dickin, M. Wang, T. Dyakowski, R.A. Forrest, P.J. Holden, Chem. Eng. Sci. 52 (1997) 2087-2097; P.J. Holden, M. Wang, R. Mann, F.J. Dickin, R.B. Edwards, A.I.Ch.E. JI. 44 (1998) 780-790]. Using the UMIST pilot plant 1.5 m stirred vessel, new results are presented for gas-liquid mixing which can distinguish differences in liquid mixing behavior between water (low viscosity) and relatively viscous 0.05% carbopol solution. The slower liquid phase tracer mixing rates for the viscous case compared with water, as well as differences in the pattern of mixing, can be readily observed and portrayed in 3-D using linear back projection for the reconstruction. In addition, the variation of local gas hold-up (voidage) can also be simultaneously measured by identifying regions within the vessel which have high or low local conductivity/resistivity. Electrical resistance tomography (ERT) can thus provide simultaneous detailed information on two aspects of gas-liquid mixing, involving the isolation of liquid phase mixing when gas-liquid mixing is taking place. The ERT results can be processed to provide the local mixing curve, hence mixing times, for any nominated pixel. (C)2000 Elsevier Science S.A. All rights reserved.