[en] The calculated critical flow-rate of a vapour liquid mixture in a pipe is very sensitive to assumptions about the rates of interphase mass and momentum transfer between the phases. To assist the development of a physically based model of these processes measurements have been made of the axial variations of pressure, void fraction and bubble number in the critical flow of subcooled water in a pipe. Experiments were performed at pressures up to 3.5 bar and temperatures up to 140⁰C, (mass velocities in the range 5-14 x 10³ kgm^-2s^-1), using pipes of 25 mm nominal bore. Measured critical flow-rates are up to a factor of two higher than predicted by homogeneous equilibrium theory but are shown to be in good agreement with predictions of a non-equilibrium two-fluid model of vapour-liquid flow. However, although this model gives a satisfactory representation of the axial profiles of pressure and density in the pipe, predicted bubble growth rates and number densities are not in good agreement with observations. Discrepancies are attributed to the role of convective heat transfer in bubble growth and to deficiencies in existing models of heterogeneous nucleation. It is suggested that despite these discrepancies the predicted non-equilibrium vaporisation rate is probably close to that found experimentally