Two-phase boiling hydrogen pressure drop is studied in the context of high velocity upflow in a constant, high heat flux, steady state, internal pipe flow environment. The approach of this analysis is to reverse-engineer the best available data to determine mass quality, void fraction, and velocity slip. This is accomplished by applying a one-dimensional, five-equation model, with pressure gradient being the one combined equation. The resulting velocity slips are correlated for high and low pressure conditions. Good agreement is achieved between the pressures predicted using the slip correlations and the measured pressures. Results are in general significantly better than those from the homogeneous equilibrium model. (c) 2007 Elsevier Ltd. All rights reserved.