Comparing the flow through an SRV to that through an orifice, a simple theory is proposed to relate the discharge coefficient in two-phase compressible flow to the value in liquid incompressible flow. This approach made use of the author's omega method applicable for both flashing discharge and non-flashing discharge. The two-phase discharge coefficient is shown to be a smooth function of the omega parameter. For non-flashing two-phase discharge, the discharge coefficient is shown to lie between the liquid coefficient and the gas coefficient. For flashing two-phase discharge, the discharge coefficient is higher than the gas coefficient. Comparison with recently published SRV data seems to support the present recommendation. (C) 2004 Elsevier Ltd. All rights reserved.