Received January 28, 1977 Accepted for Publication September 7, 1977 The effect of unequal vapor and liquid velocities has been introduced into the conservation equations in a manner that is consistent with the use of the method of characteristics. It is shown that these reformulated equations have real characteristics if realistic slip ratios are used. The low slip ratios needed to ensure real characteristics at high mass flow rates are in conformity with the view that the slip ratio must approach 1.0 at these conditions. Application of the computational procedure to the estimation of critical flow rates leads to predictions in agreement with the homogeneous-equilibrium model when thermodynamic equilibrium is assumed.