The nature of the active species in a dehydration reaction using the “catalyst” Re2O7 is investigated. During studies on the cleavage of the β-O-4 bond of a lignin model substrate, apparent “TOFs” of 240 h−1 and good selectivities of up to 73% for the sensitive product phenylacetaldehyde (compared to other applied (Lewis) acids) are observed. However, divergent kinetic data during the studies led to an in-depth investigation on the activation mechanism of Re2O7 in the applied apolar, aprotic medium. Here, it is shown, that, in fact, Re2O7 itself is completely inactive in the dehydration reaction, but forms even at low temperatures mixed valence rhenium nanoparticles, which promote the desired reaction. The nanoparticles are characterized by XPS, DLS and TEM. In order to discriminate whether either dissolved rhenium compounds or nanoparticles are the active species, 17O NMR studies and kinetic experiments are performed. An activation mechanism of rhenium heptoxide is suggested based on a careful analysis of byproducts under catalytic conditions and application of selected potential catalysts. Furthermore, an outlook is given, as these nanoparticles might participate in numerous other catalytic reactions.