Thin films of Ti1-xZrxN were deposited on silicon substrates by reactive r.f. sputtering of a segmented Ti-Zr target in an Ar + N-2 discharge. The crystal structure and microstructure of the films deposited under various negative substrate bias voltages were studied by X-ray diffraction (XRD) and field emission scanning electron microscopy. The determination of composition was carried out by the measurement of Rutherford backscattering spectrometry (RES), and the porosity of the films was evaluated quite quantitatively based on the data of both the apparent thickness measured by surface profilometer and the mass thickness obtained from RES and XRD. An interstitial crystal structure model was introduced for the porosity calculation, and the possibility of oxide formation for Ti and/or Zr in the films containing high oxygen concentration was taken into account in the porosity evaluation. The bias effect on the porosity, oxygen impurities, internal stress and resistivity of the deposited films was studied in detail.