Tungsten pentoxide () was reported in early literature but proved to have the stoichiometry W<sub>18</sub>O<sub>49</sub>. Sometimes called mineral blue, it is a blue solid formed by the reaction of tungsten trioxide (WO<sub>3</sub>) and tungsten metal at 700 ðC.
There are a number of these unusual intermediate oxides formed from reacting metal and trioxide, including W<sub>20</sub>O<sub>58</sub> and W<sub>24</sub>O<sub>70</sub>. W<sub>18</sub>O<sub>49</sub> contains both octahedral and pentagonal bipyramidal co-ordination of the metal atoms by oxygen.