Subsolidus phase relations have been determined for the Bi-Mn-Nb-O system in air (750-900 1C). Phases containing Mn 2+ , Mn 3+ , and Mn 4+ were all observed. Ternary compound formation was limited to pyrochlore (A 2 B 2 O 6 O 0 ), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi 2 (Mn,Nb) 2 O 7 ) suggesting that E14-30% of the A-sites are occupied by Mn (likely Mn 2+ ). X-ray powder diffraction data confirmed that all Bi-Mn-Nb-O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi 2 O 3 :Mn 2 O 37x :Nb 2 O 5 using neutron powder diffraction data is reported with the A and O 0 atoms displaced (0.36 and 0.33 A ˚, respectively) from ideal positions to 96g sites, and with Mn 2+ on A-sites and Mn 3+ on B-sites (Bi 1.6 Mn 2+ 0.4 (Mn 3+ 0.8 Nb 1.2 )O 7 , Fd3m (]227), a ¼ 10:478ð1Þ A ˚); evidence of A or O 0 vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi 1.5 Zn 0.92 Nb 1.5 O 6.92 , which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent with an Mn oxidation between 2+ and 3+. Bi-Mn-Nb-O pyrochlores exhibited overall paramagnetic behavior with negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spinonly values. At 300 K and 1 MHz the relative dielectric permittivity of Bi 1.600 Mn 1.200 Nb 1.200 O 7 was E128 with tan d ¼ 0:05; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Low-temperature dielectric relaxation such as that observed for Bi 1.5 Zn 0.92 Nb 1.5 O 6.92 and other bismuth-based pyrochlores was not observed. Bi-Mn-Nb-O pyrochlores were readily obtained as single crystals and also as textured thin films using pulsed laser deposition.