Within the current work, niobium oxide thin films have been deposited by reactive d.c. magnetron sputtering from cold gas sprayed niobium targets, applying a systematic variation of oxygen partial pressure, bias pulse frequency and substrate temperature. The films grown without additional substrate heating are amorphous. X-ray photoelectron spectroscopy shows the Nb5+ oxidation state, characteristic for the Nb-2O5 phase, for films grown at the highest oxygen partial pressure applied, while the Nb4+ state of NbO2 is additionally present at lower oxygen partial pressures. With increasing oxygen partial pressure, an optical shift from non-transparent to nearly fully transparent was observed, whereby the higher bias pulse frequency leads to a transition already at lower oxygen partial pressures. The transparent films are characterized by a maximum transparency of similar to 80% and a refractive index of similar to 2.5. Increasing the substrate temperature up to 600 degrees C results in the formation of well crystalline coatings consisting of the orthorhombic Nb2O5 phase, which are whitish and semi-transparent.