Abstract Pure phase B1 NbN layers could be sputtered in our laboratory only with a maximum T c of 15.9 K and a residual resistance ratio RRR of <0.9. Samples with a T c of up to 16.7 K invariably had a large fraction of hexagonal foreign phase. If under similar conditions one sputters layers in the presence of small amounts of Al, one obtains pure B1 phase NbN layers with a maximum T c of 16.9 K. The RRR rises to values of up to 1.16. The specific resistance drops from typical values of around 400 μωcm to values as low as 73 μωcm. The width of the X-ray lines drops, indicating increased grain size. Al acts like a catalyst, being substituted with less than 0.03 at% for the high T c layer. When Al is substituted for Nb in detectable quantities both T c and the lattice parameter drop. To verify the above results the experiment was repeated in a magnetron rather than a RF sputter system with similar outcome.