Density functional calculations reveal a new mechanism for the decomposition of rhenium alkyl azide. PhN3 binds to the rhenium center in an n(1)-diazoamino mode ([Re] N(Ph) N N) rather than the more common n(1)-terminal mode ([Re] N N N Ph). From this new intermediate direct toss of N-2 is fast, while the previously postulated cyclic 4-centered intermediate/transition state are very unfavorable in this system.