The gas phase reaction of Zn and H2O in a He carrier gas has been used as the basis for the chemical vapor deposition of ZnO on sapphire and MgAl spinel. Deposit characteristics were studied as a function of reactor linear gas stream velocity, Zn/H2O vapor phase ratio, temperature and substrate preparation. It was round that the substrate support can influence the surface morphology significantly and that in situ pretreatment can affect epitaxial relationships between deposit and substrate. Transparent, visually smooth deposits of (11-24) ZnO can be obtained on chemically polished (0001) sapphire at 815°C using average linear gas stream velocities, ν, of 6-12 cm/sec referenced to room temperature in conjunction with a Zn/H2O reactor input-pressure ratio of 0.02-0.09 (using Zn reactor pressures of 1.2-5.0 × 10-3 atm) . The substrates are given an H2O in situ pretreatment at 900°C prior to deposition at ν = 3 cm/sec with the partial pressure of H2O in He = 5.7 × 10 atm. © 1973 American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.