The temperature of GaAs substrates is profiled in a molecular-beam epitaxy system with a spatial resolution of 3 mm and a thermal resolution of 0.4-degrees-C, respectively. The effects of substrate doping, back surface textures, thermal contact to the holder, and a pyrolytic boron nitride diffuser plate, on the temperature uniformity, are explored for indium-free mounted substrates. Both positive and negative curvature temperature profiles are observed.