Semirigorous equations are developed for flow of flashing liquids in pipe lines. These relations are applied to the flow of hydrogen, and computed correlations are presented for the pressure drop and vapor fraction. The calculations have been carried out as functions of the parameters: diameter, 3/8 to 4 in.; mass rate per unit area, 0.0528 to 682 lb./(sq. ft.) (sec.); pressure, 14 to 30 lb./sq. in. abs.; vapor fraction, 0.005 to 0.65; and heat leak, equivalent to 0 to 0.0224 B.t.u./(sq. ft.) (sec.) for a 3/4-in. line.