Steam-water, two-phase critical flows were obtained in long pipes (L/D > 40) for mass flow rates ranging from 512 to 6460 lbm/(sec ft2), exit pressures from 40 to 150 psia, and thermodynamic equilibrium qualities from 0.0019 to 0.216. A comparison of the three test sections employed indicates that previous experimental data are in error for qualities less than 0.10 due to the influence of the downstream two-dimensional expansion on wall pressure taps located near the exit plane. Although simultaneous temperature and pressure measurements were not taken, the data exhibit trends that suggest the existence of a nonequlibrium phase change. Experimentally determined exit and axial void fractions indicate (a) that the velocity ratios are considerably less than the existing analytical predictions and (b) that previously dissolved gases existing from the liquid provide a source for vapor formation under adiabatic subcooled conditions.