An experimental set-up for the measurement of steady-state two-phase flow data at sub-atmospheric pressures has been built. It has been used to provide data for boiling water. Void fraction data showed a distinct mass quality dependence but only a weak mass flux dependence. Friction pressure drop was calculated from the measured total pressure drop. A clear dependence was found on mass quality and on mass flux. Quasi-local heat transfer coefficients were calculated from temperature and heat flow measurements. Nucleate boiling was found to dominate the heat transfer, although the convective contribution was evident. Empirical correlations for all the quantities were derived.