Effective and selective capture of environmentally toxic CrO from water is desirable for both environment protection and human health, but it still remains a significant challenge. We present a water-stable cationic metal-organic framework (MOF) with large nanotubular channels ( ca. 1.4 × 1.4 nm), (1-Cl). Remarkably, the resulting porous material exhibits rapid aqueous-phase removal of CrO via an anion-exchange manner. Meaningfully, the capture and separation of aqueous CrO are highly selective even in the presence of other disturbing anions. More importantly, the crystal structure of 1-Cl after anion exchange (1-CrO) could be determined by single crystal X-ray diffraction, elaborating the single-crystal-to-single-crystal (SC-SC) transformation. The CrO removal process by 1-Cl thus was directly uncovered by the crystal structure of CrO-incorporated 1-CrO.