Continuous food-sterilisation processes involve the flow of solid liquid mixtures in pipes. Prediction of particle passage times in the system is required for ensuring sterility and optimising product quality. It is important to be able to predict both the minimum and maximum passage times of particles in the heating and holding sections of the system, and ideally the whole distribution of passage times (PTD) should be known. A Positron Emission Particle Tracking (PEPT) technique was used to determine the trajectories of almost neutrally-buoyant 5-10 mm alginate spheres in viscous non-Newtonian solutions. Particle passage times were measured by Hall effect sensors or visual tracers. A wide range of experimental conditions were investigated including solids fractions from 16 to 55 vol% and mean mixture velocities from 20 to 230 mm s(-1). Thus, the mean apparent viscosity of the carrier fluid ranged from 20 to 510 mPas corresponding to tube Reynolds numbers of 2.1-381. Experimental results revealed the existence of four different PTD forms depending on the flow pattern present. A theoretical two-region flow model is presented which gives good predictions of the PTD forms, the minimum and maximum particle passage times. (C) 2003 Elsevier Science Ltd. All rights reserved.