Refluxing of the red compound [CuIIL1C(H 2O)] (1) in acetone forms two new species: green [Cu IIL1JH2O)] (2) and brown [CuIIL 1] (3) [H2L1 = N,N'-ethylenebis(3- ethoxysalicylaldirnine)]. Whereas both 1 and 2 are converted into 3 upon heating, 3 is converted back into 1 when exposed to air. The structures of all three compounds were determined crystallographically, from which the interesting hydrate isomerism of 1 and 2 are identified; in 1 a water molecule is encapsulated into the O4 section of the ligarid through bifurcated hydrogen bonding, whereas in 2 a water molecule coordinates directly to the metal centre. Despite the metal ion binding to the ligand in the same manner in all three cases, the formation of different mononuclear complexes resulting from the pres ence or absence of water and its coordination or encapsulation is an interesting observation. Thermogravimetric analyses of compounds 1 and 2 showed that the encapsulated water in the former compound is removed at slightly lower temperature than that of the coordinated water molecule, as expected. Although three crystals 1, 2 and 3 were isolated, the composition of species in solution is probably the same as evidenced by 1H NMR spectroscopy. Therefore, the formation of three crystals and the hydrate isomerism, described in this investigation are solid-state phenomena. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.