The experimentally determined quantitative effects of varying gas mixture composition on the properties of the zirconium carbide (ZrC) deposited on microspheres in a fluidized bed were a decrease in metallic appearance of the ZrC coat, with an increase in the ratio of the hydrocarbon gas to the ZrCl4 and co-deposition of carbon at high hydrocarbon gas concentrations. Increasing the H2 concentration inhibited these effects and permitted the ZrC to be deposited at higher hydrocarbon gas concentrations. Deposits of pure sub-stoichiometric ZrC coats were controllable over a limited concentration range. The ZrC was deposited in a fluidized bed of ThO2 particles at a maximum temperature of ∼1650 K, using gas mixtures of H2, argon, ZrCl4, and CH4 or C3H6. The ZrCl4 flow was controlled using a powder feeder.