Abstract Transition metal acetylide CoC2 is highly absorbent of water due to its ionic character. In the anhydrous phase, CoC2 forms NaCl-like cubic structure where the orientation of C 2 2 - dianions are disordered. Once the material is exposed to air or water, the lattice of CoC2 is expanded anisotropically by the absorbed water, which restricts the orientation of C 2 2 - dianions. The magnetism of CoC2 also changes drastically by air/water exposure owing to the orientation ordering of C 2 2 - dianions. The anhydrous CoC2 behaves as superparamagnet down to 1.8 K because the magnetic domains of CoC2 are cut into pieces by orientation disorder of C 2 2 - dianions, while the hydrous CoC2 shows the ferromagnetism thanks to the large ferromagnetic domains caused by the orientation ordering of C 2 2 - dianions.