We report the use of diffuse reflectance spectroscopy for active, closed-loop control of substrate temperature during the growth of a modulation doped heterostructure. Measurement and control of substrate temperature is a common difficulty for molecular beam epitaxy (MBE), as well as other semiconductor deposition techniques. To examine the effect of the temperature lag experienced during conventional MBE, we have grown identical pairs of GaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistors (pHEMTs). For one pHEMT in each pair, the input signal for substrate temperature control was the standard thermocouple (TC); fur the other, it was diffuse reflectance spectroscopy (DRS). Under TC control, an overshoot of up to 70 degreesC was observed during the temperature upramp following the lower-temperature deposition of the InGaAs layer. This overshoot was eliminated under DRS control. Temperature errors can lead to heterolayer changes such as segregation, desorption, and changes in dopant activation. [S0734-211X(00)01205-0].