New tetradentate Schiff base-oxovanadium(IV) complexes, [VO{3-Xsal-(R,R)-2,4-ptn}] (H-2{3-Xsal(R,R)-2,4-ptn} : N,N’-di-3-Xsalicylidene-(R,R)-2,4-pentane X = EtO (ethoxy), MeO (methoxy), and H) were prepared and characterized. Only an orange polymeric form was obtained for the 3-methoxy and nonsubstituted (X = H) complexes. Both orange polymeric (1) and green monomeric (2) crystals were obtained for the S-ethoxy substituted complex. X-ray structure analyses of 1 and 2 were carried out. Crystal data : [VO{3-EtOsal-(R,R)-2,4-ptn}]. H2O (1), VO6N2C23H30, orthorhombic, P2(1)2(1)2(1), a = 13.764(4) Angstrom, b = 21.544(4) Angstrom, c = 7.682(5) Angstrom, Z = 4; [VO{3-EtOsal-(R,R)-2,4-ptn}] (2), VO5N2C23H28, orthorhombic, P2(1)2(1)2(1), a = 10.393(2) Angstrom, b = 22.278(2) Angstrom, c = 9.886(2) Angstrom, Z = 4. The orange crystals (1) have water of crystallization, and the complexes take a polynuclear linear chain structure (... V=O ... V=O ...) with a six-membered N-N chelate ring in a chair form. The green crystals (2) have no water of crystallization, and the complexes take a mononuclear square-pyramidal structure with a six-membered N-N chelate ring in a flattened boat form. Upon heating at 170 degrees C for 10 min, orange 1 turned to green 2, which reverted to orange 1 by suspension in water-acetonitrile. Mechanism of the interconversion between the polymeric and monomeric forms is discussed on the basis of their crystal structures.