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 degrees C. Observed phases were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, combustion analysis, and thermogravimetric analysis. An Nb4N5-related phase, with a metal content of less than or equal to 15 at% Mn, was obtained pure at 650-700 degrees C. A new hexagonal phase MnNb2(N,O)(3) was obtained at 750-900 degrees C, Its structure is related to, but not isotypic with, that of Nb5N6. In addition, the preparations contained cubic NaCl-type (Mn, Nb)(N,O) phases. The Mn-free Nb4-y(N,O)(5) and Nb5-y(N,O)(6) (y signifying metal vacancies), with Nb4N5- and Nb5N6-type structures, respectively, were synthesized, to make possible structural comparisons with the Mn-containing phases. A black trigonal phase Mn-3.66(6)Nb-2.34(6)(O,N)(9) with a structure closely related to that of Mn4Nb2O9 was obtained at 700 degrees C.