Rare earth doping is the key strategy to increase the Curie temperature (TC) of the ferromagnetic semiconductor EuO. The interplay between doping and charge carrier density (n), and the limit of the TC increase, however, are yet to be understood. We report measurements of n and TC of Gd-doped EuO over a wide range of doping levels. The results show a direct correlation between n and TC, with both exhibiting a maximum at high doping. On average, less than 35% of the dopants act as donors, raising the question about the limit to increasing TC. © 2010 The American Physical Society.