Polyfluoroxometalates (PFOMs) that have a quasi Wells-Dawson structure and have low valent transition metal substitution at the so-called "belt" position, α 1 -[H 2 F 6 NaM-(H 2 O)W 17 O 55 ] q-, can reversibly interchange between dimeric and monomeric structures. The dimers have two unique M-μO-W bridges between two PFOM units. The dimerization occurs through dehydration and was studied as a function of temperature using the visible spectrum that is sensitive to the wavelength and extinction coefficient of the d-d transition. The [a]