Phase formation in the system Mn-Nb-O-N has been studied by ammonolysis of mixtures of Mn acetate tetrahydrate and an Nb xerogel at 600-900 o C. Observed phases were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, combustion analysis, and thermogravimetric analysis. An Nb 4 N 5 -related phase, with a metal content of =<15 at% Mn, was obtained pure at 650-700 o C. A new hexagonal phase MnNb 2 (N,O) 3 was obtained at 750-900 o C. Its structure is related to, but not isotypic with, that of Nb 5 N 6 . In addition, the preparations contained cubic NaCl-type (Mn,Nb)(N,O) phases. The Mn-free Nb 4 - y (N,O) 5 and Nb 5 - y (N,O) 6 (y signifying metal vacancies), with Nb 4 N 5 - and Nb 5 N 6 -type structures, respectively, were synthesized, to make possible structural comparisons with the Mn-containing phases. A black trigonal phase Mn 3 . 6 6 ( 6 ) Nb 2 . 3 4 ( 6 ) (O,N) 9 with a structure closely related to that of Mn 4 Nb 2 O 9 was obtained at 700 o C.