The development of a hybrid model for predicting outflow following the rupture of downward sloped pressurized hydrocarbon pipelines is described. The model addresses the failure of the Method of Characteristics (MOC)-based numerical solution technique in predicting post depressurization outflow through coupling it with a hydraulic flow model. The resulting hybrid simulation is tested against the hypothetical full bore rupture of a 100-m-long, 0.158-m-diameter pipeline at various angles of decline in the range 0 to -90 degrees containing different classes of hydrocarbons. These include a two-phase mixture, flashing liquid, liquid, and a permanent gas. The results indicate that in the case of the two-phase mixture and the permanent gas, both the MOC and the hybrid models produce very similar results with outflow being independent of the angle of decline. However, large differences in the data are obtained in the case of liquid and flashing liquid inventories with the MOC significantly under predicting outflow. (C) 2008 American Institute of Chemical Engineers.