If a gas-pressurised vessel is ruptured, the thrust exerted on it as a reaction to the ejection of gas through the developing breach will accelerate the vessel. A simple model has been developed to predict the velocity achieved by an unrestrained vessel when rupture is initiated by an axial split. The predicted velocity is derived from an approximation to the impulse applied to the vessel. A comparison of the predicted velocity with data from a parallel experimental investigation shows that the simple model provides a realistic upper bound to the observed velocities of both whole vessels and large fragments generated in instances where vessel break up occurred. (C) 2001 Elsevier Science Ltd. All rights reserved.