Particle motion in a vertical-axis mixer was studied using discrete element method (DEM) simulations and positron emission particle tracking experiments. The mixer was fixed with a circular disk rotating in a horizontal plane or a simple paddle. In the DEM simulations, linear springs, dash-pots and frictional sliders were used to model the contact mechanics between the particles and the particles and the walls. Quantitative comparisons were made between the numerical calculations and the experimental measurements. The DEM prediction captures the major features of the flow patterns in the mixer when the mixer was fixed with a disk impeller rotating at 100 r.p.m., although the predicted particle velocities are higher than experimental measurements when using physically reasonable simulation parameters (normal stiffness = 1000 and 10 000 N/m; coefficient of restitution = 0.9; internal friction coefficient = 0.2, 0.3 and 0.45; wall friction coefficient = 0.2, 0.25 and 0.3). However, when the mixer was fixed with the paddle impeller, the calculated results using physically reasonable simulation parameters were different from the measurements. The calculated particle velocity was as high as 2 m/s, while the averaged particle velocity from measurement was about 0.1 m/s.