Blue prismatic crystals of mononuclear [Cu II (O 2 CH) 2 (Py) 3 ] (Py = pyridine, C 5 H 5 N), 1, were prepared from reaction solution containing dry pyridine and dinuclear [Cu 2 II (O 2 CH) 4 (Py) 2 ]. When a portion of solution together with crystals was exposed to air moisture, crystals of 1 dissolved in mother liquid. Simultaneously, blue needles of 2, i.e. covalently linked one-dimensional chain structure with formula [Cu(O 2 CH) 2 (Py) 2 ] n • nH 2 O, grew out of the solution. The process lasted few minutes and was observed under optical microscope. The conversion from 1 to 2 takes place also outside of solution in the solid state. Single crystal X-ray diffraction data were collected at 150 K for 1 and after that for 2, originating from the same reaction solution. This paper reports the structures of both compounds. Alternative synthesis method of 2 arising from a mixture of copper methanoate and pyridine is also reported.