Polymorphic phase transitions remain frequently undetected in routine metal organic framework (MOP) studies; however, their discovery is of major importance in interpreting structure property relationships. We herein report a reversible enantiotropic single-crystal to single-crystal polymorphic phase transition of a new microporous MOP [Eu(BDC)(NO3)(DMF)(2)](n)(H2BDC = 1,4-benzenedicarboxylic acid; DMF = dimethylformamide). While modification 1LT at 170 K crystallizes in the monoclinic space group P2(1)/c with unit cell dimensions of a = 17.673(2) angstrom, b = 20.023(2) angstrom, c = 10.555(9) angstrom, beta = 90.129(4)degrees, modification 1HT at 290 K crystallizes in higher symmetry space group C2/c with unit cell dimensions of a = 17.200(7) angstrom, b = 10.737(4) angstrom, c = 10.684(4) angstrom, beta = 90.136(2)degrees. This temperature-induced phase transition is accompanied by a small change in the solvent-accessible voids from 46.8 in 1LT to 49.8% in 1HT, which triggers a significant change in the adsorption properties as compared to a reported isostructural compound. Detailed investigations on the phase transition were studied with variable-temperature single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction, and differential scanning calorimetry measurements. The herein-presented investigations emphasize the importance of polymorphic phase transitions in routine MOP studies originating from low-temperature SCXRD data and high-temperature physical property characterizations in avoiding the use of a wrong structure in interpreting structure property relationships.