This paper describes the preparationand characterization of nanoscaledM-MOF-74/CPO-27-M (M = Mg, Mn, Co, Ni, and Zn) materials at room temperature.Some of the so-formed crystals are the smallest ones of any metal–organicframework (MOF) material (and, to the best of our knowledge, of anymicroporous material) ever reported. They are at the limit of beingable to diffract, particularly those forming the Co- and Ni-MOF-74samples. Consequently, unequivocal identification as the crystallineMOF-74 phase was deduced by combining other characterization techniquesrather than powder X-ray diffraction. These small crystals are unstableas isolated ones, so they form steady and robust aggregates, whosemechanical properties strongly depend on the crystal size. The particlesthat result from the “fusion” of nanocrystals smallerthan 10 nm (more properly denoted as nanodomains) could not be disaggregatedby conventional ultrasonic and graining techniques. On the contrary,agglomerates of crystals larger than 10 nm are dissociable in discretecrystals. It allows characterizing Zn-MOF-74 nanocrystals by advancedelectron microscopy methods. Cs-corrected scanning transmissionelectron microscopy (STEM) provided, for the first time, “quasi”atomic resolution images of MOFs, which are especially unstable underelectronic radiation. The magnitude of the crystal size of M-MOF-74is tentatively associated with the solubility of the metal source. [ABSTRACT FROM AUTHOR]