A new model to calculate the two-phase flow pressure change across a sudden expansion in a duct area was developed and checked against data measured with mixtures of air and water, aqueous glycerol, watery calcium nitrate and with refrigerant R 12. In the model all relevant physical parameters are included and, in contrast to equations in the literature, the entrainment of liquid in the gas stream is considered. The predictions are validated for a wide range of conditions, pipe diameters and physical properties typically encountered in industrial pipe line systems. Calculations based on the new approach are sufficiently accurate for engineering purposes.