The Martinelli model for pressure drop in flowing two-phase systems has been examined in detail for para-hydrogen from 1 atm. to its critical pressure. Amethod for obtaining the Martinelli Φ term, two phase friction factor, at intermediate pressures is presented. Φ2 can be expressed as a function of pressure and quality. The Martinelli β term, ratio of actualvapor or gas area to an idealized cylindrical area, is found to vary greatly and to indicate first an increase in gas-liquid interface to gas volume ratio and then a decrease with increasing pressure. Some experimental data are examined in light of this development.