The aim of this paper is to give an idea of how much the lateral stability of a road tanker is affected by the liquid load motion. Laboratory tests with a physical/mathematical model of the tank and vehicle chassis is presented, showing that liquid movements in the tank can destabilize the vehicle severely. A vehicle, loaded with less than half the tank volume may be remarkably more unstable than when fully loaded - in spite of its lower centre of gravity. The problem might be even more pronounced in real-life since the liquid and roll resonance frequencies are close to each other in conventional road tankers. The most desirable improvement is complete prevention of the liquid motion, i.e. by applying a horizontal wall at the liquid surface and make it follow the surface when the tank is being loaded or unloaded. Until appropriate construction techniques have been established, the liquid motion can be decreased by installing vertical walls in the tank and keeping a minimum number of tank compartments partly loaded.