This paper describes the development and validation of a highly efficient robust numerical simulation based on the method of characteristics for predicting release rates following the puncture of pipelines containing high pressure hydrocarbon mixtures. The model accounts for real fluid behaviour, radial and axial flow as well as the locality of puncture relative to the length of the pipeline. It is applicable to both isolated and un-isolated flows where pumping at the high-pressure end continues despite puncture. The results of the application of the model to the hypothetical puncture of a 16 km, 0.42 m dia. pressurised pipeline containing a condensable hydrocarbon mixture are presented as a case example. Simulated fluid velocity and pressure profiles are used to provide a pictorial timeline representation of the important post-puncture fluid dynamics phenomena within the pipeline, which ultimately govern the discharge process. These results indicate that the conventional outflow models treating the pipeline as a vessel discharging through an orifice are inappropriate especially during the early stages of depressurisation. Good agreement is obtained between the results of the simulation when compared to experimental data from the puncture of 100 m long LPG pipeline. (C) 2003 Elsevier Ltd. All rights reserved.