A mathematical model has been considered in which the influence of gas release on transient cavitating flow and column separation in pipel ines is taken into account. A rei iable numerical method has been developed for the computation of the wave propagation and cavitation phenomena following pump fai lure, valve closure, etc. Good agreement has been obtained with avai lable experimental results concerning column separation. Including gas release in the theory has no great influence in cases where only cavitating flow occurs, whereas the influence is considerable in cases where column separation together with cavitating flow occurs. Gas release in the cavitating flow region adjacent to a column separation cavity, which region wi I I occur if the waves are steep (e.g., sudden valve closure), diminishes the duration of the subsequent column separations and the maximum pressures following column separation. The related energy loss can be attributed to dissipation caused by shock waves propagating in the bubble mixture. Gas release at the separation cavity has a different effect: the duration of column separation increases, and the pressures increase slightly. The former process is sensitive to the amount of gas released, whereas the latter isnot.