Experimental data on steam void fraction and axial temperature distribution in an annular boiling channel for low mass-flux forced and natural circulation flow of water with inlet subcooling have been obtained. The ranges of variables covered are: mass flux = 1.4 × 104-1.0 × 105 kg/hr m2; heat flux = 4.5 × 103-7.5 × 104 kcal/hr m2; and inlet subcooling = 10-70°C. The present and literature data match well with the theoretical void predictions using a four-step method similar to that suggested by Zuber and co-workers. © 1975.