The solid solution of vanadium in hexagonal molybdenum oxide has never previously been completely studied; no information has been given about the solubility limit or the thermal stability of the system. In this work, we have determined that the solubility limit is reached with the following composition: (NH 4 ) 0.13 V 0.13 Mo 0.87 O 3 . Our samples have been obtained by a soft chemistry method. The obtained precipitates have been processed and analyzed with several techniques. We have shown that the introduction of vanadium in the h-MoO 3 structure has several effects, on the thermal stability of the metastable mixed oxide, on the morphology, as well as on the lattice parameters.