We study the deposition of Si and Nb oxides by reactive d.c.-magnetron sputtering from high temperature metallic Si and Nb targets in Ar/O-2 atmospheres. It is shown that the oxides formed on Si targets above 1 500 K and on Nb above 2 310 K evaporate, leaving the target surface free of the solid oxide coatings which induce hysteresis, voltage instability and are breakdown in conventional reactive sputtering. Also, the deposition rate can be much higher because the target surface is coating-free and because sublimed oxides, SiO(g) for Si targets and a mixture of NbO(g) and NbO2(g) for Nb, contribute significantly to the deposit : The deposition rate of SiO2 is three times higher than for a conventional process with the same power and for Nb2O5, it is seven times higher. Thermionic emission is shown to be an important process which can constitute more than half of the total discharge current in the hot target sputtering of Nb.