Detonation cell widths of hydrogen/oxygen/helium/steam mixtures were determined in 15 and 30-cm internal-diameter pipes using the smoked-foil technique. The investigations were carried out over a range of initial pressures from 13.3 to 106.6 kPa and at an initial temperature of 105 degrees C. The concentration range investigated included hydrogen concentrations from 10 to 40 percent and steam concentrations of up to 20 percent. The measured cell widths range from about 1 mm to over 100 mm and were in good agreement with those estimated using the calculated values of induction times and postshock gas velocities.