This paper describes the development and experimental validation of a three-phase flow model for predicting the transient outflow following the failure of pressurised CO2 pipelines and vessels. The choked flow parameters at the rupture plane, spanning the dense-phase and saturated conditions to below the triple point, are modelled by maximisation of the mass flowrate with respect to pressure and solids mass fraction at the triple point. The pertinent solid/vapour/liquid phase equilibrium data are predicted using an extended Peng-Robinson equation of state. The proposed outflow model is successfully validated against experimental data obtained from high-pressure CO2 releases performed as part of the FP7 CO2PipeHaz project (www.co2pipehaz.eu). The formation of solid phase CO2 at the triple point is marked by a stabilisation in pressure as confirmed by both theory and experimental observation. For a fixed diameter hypothetical pipeline at 100 bar and 20 degrees C, the flow model is used to determine the impact of the pipeline length on the time taken to commence solid CO2 discharge following its rupture. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier BAT. All rights reserved.