We have examined the problem of substrate temperature changes during the growth of YBa2Cu3O7−x (YBCO) thin films on LaAlO3 when the substrate is radiatively heated. Substrate temperature variations sufficient to degrade film quality occur unless the heater temperature is controlled during growth. At a heater temperature of 900 °C, the temperature of a LaAlO3 substrate was measured to be only 540 °C, while a similar substrate with a 2000‐A YBCO film reached 640 °C. The experimental data and calculations indicate that the heater temperature must be as high as 1240 °C to heat LaAlO3 to 730 °C, and must be decreased by over 200 °C during the first 500 A of film deposition to maintain a constant substrate temperature during film growth. This study shows the need for an in situ noncontact substrate temperature measurement technique.