The phase transformation and magnetic properties of manganese bismuthide have been reinvestigated with single crystal samples of low temperature phase (LTP) and high temperature phase (HTP) in the quenched state (QHTP). It was established that the LTP and HTP are separate compounds with the chemical formulae MnBi and Mn1.08Bi respectively. The ferro-to-paramagnetic transition upon heating to 628 K and para-to-ferromagnetic transition upon cooling to 613 K correspond respectively to the phase decomposition of MnBi → Mn1.08Bi + Bi and Mn1.08Bi → MnBi + Mn. The effects of these transformations were also investigated in thin film specimens and the distribution of each phase in the thin films was determined. Magnetic properties of the LTP, HTP and QHTP obtained from the single crystals were compared to the previous data obtained from polycrystalline samples or thin films. For the QHTP single crystals, the saturation magnetization, Ms, at 4 K is 3.04 μBper Mn atom. This is 86% of the Msof the LTP. At 300 K we find Msto be 1.8 times larger than previously reported. The anisotropy energy K in the QHTP was found to be much larger than that of the LTP.