The predictive performance of existing models based on momentum balances has been shown to be generally unsatisfactory. An iterative procedure was developed for the prediction of pressure drop and holdup that incorporated new relationships for the interfacial and liquid friction factors in the solution of the phase momentum balance equations for two phase horizontal co-current Bow. The method adequately predicted data for the film plus droplet, annular roll wave and stratified type regimes. Successful performance was achieved regardless of fluid properties or pipe diameter. For gas flow rates, where nonuniform stratified Bow with an interfacial lever gradient occurred, this method of prediction was inaccurate and open channel flow theory recommended. Thus the model did not apply to the intermittent or nonuniform flow regimes.