We deposited Ta-N films by reactive r.f. sputtering from a Ta target with an N-2,-Ar gas mixture. Alloys over a composition range 0-60 at.% N have been synthesized. We report on their composition, structure and electrical resistivity before and after vacuum annealing in the temperature range 500-800 degrees C. We found that the film growth rate decreases with increasing ratio of the nitrogen flow rate to the total Aow rate, while the nitrogen content in the films first increases with the N-2 partial flow rate and then saturates at about 60 at.%. B.c.c.-Ta, Ta2N TaN and Ta5N6 appear in succession as the nitrogen content rises, with Ta2N being the only single-phase film obtained. The atomic density of the films generally increases with the nitrogen content in the film. Transmission electron micrographs show that the grain size decreases from about 25 to 4 nm as the nitrogen concentration increases from 20 to 50 at.%. The Ta2N phase can exist over a wide range of nitrogen concentration from about 25 to 45 at.%. For as-deposited films an amorphous phase exists along with polycrystalline Ta2N in the center portion of that range. This phase crystallizes after vacuum annealing at 600 degrees C for 65 min. A diagram of stable and metastable phases for Ta-N films based on X-ray diffraction and transmission electron microscopy results is constructed. The resistivity is below 0.3 m Omega cm for films with 0-50 at.%N and changes little upon vacuum annealing at 800 degrees C.