A method is reported for controlling substrate temperature during chemical vapor deposition of diamond in high heat flux environments such as thermal plasmas or flames. The method is based on flowing a variable-composition argon-helium mixture through channels between the substrate and a water-cooled base. A fiberoptic pyrometer temperature reading from the back of the substrate provides feedback for active substrate temperature control. Experiments in an atmospheric-pressure RF reactor under diamond growth conditions indicated that substrate temperature could be sensitively varied by over 600 K by varying the composition of the argon-helium mixture. The effect is explained in terms of the variable thermal contact resistance at the interface between the substrate and the base.