Rare earth doping is a standard, yet experimentally poorly understood method to increase the Curie temperature (TC) of the ferromagnetic semiconductor EuO. Here, we report on the charge carrier density (n) and the TC of commonly used 4.2 at. % Gd-doped EuO films grown by molecular-beam epitaxy on (110) oriented YAlO3 substrates at various substrate temperatures (Tsub). Increasing Tsub leads to a decrease in n and TC. For high substrate temperatures the Gd-doping is rendered completely inactive: n and TC drop to the values of undoped EuO. © 2011 American Institute of Physics.