The manganese-nitronyl-nitroxide two-dimensional coordination polymer {[Mn-2(NITIm)(3)]ClO4}(n) (1) (NITImH = 2-(2-imidazoly1)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxide-1-oxyl) undergoes an unusual hysteretic thermo-induced valence tautomeric transition near room temperature, during which the manganese(II) ions are oxidized to manganese(III) and two of the three deprotonated radicals (NITIm(-)) are reduced to their diamagnetic aminoxyl form (denoted NITRed2-). Upon cooling, the high temperature species {[Mn-2(II)(NITIm)(3)]ClO4}, (1(HT)) turns into the low-temperature species {[Mn-2(III)(NITRed)(2)(NITIM)] ClO4}(n) (1(LT)) around 274 K, while on heating the process is reversed at about 287 K. This valence tautomeric phenomenon is supported by temperature-dependent magnetic susceptibility measurements, differential scanning calorimetry (DSC), crystal structure determination, UV-vis absorption, X-ray absorption (XAS), and emission (XES) and electron paramagnetic resonance (EPR) spectroscopies in the solid state.