  Polycrystalline CuMo$_{1-x}$W$_x$O$_4$ solid solutions were studied by
resonant X-ray emission spectroscopy (RXES) at the W L$_3$-edge to follow a
variation of the tungsten local atomic and electronic structures across
thermochromic phase transition as a function of sample composition and
temperature. The experimental results were interpreted using ab initio
calculations. The crystal-field splitting parameter for the 5d(W)-states was
obtained from the analysis of the RXES plane and was used to evaluate the
coordination of tungsten atoms. Temperature-dependent RXES measurements were
successfully employed to determine the hysteretic behaviour of the structural
phase transition between the $\alpha$ and $\gamma$ phases in
CuMo$_{1-x}$W$_x$O$_4$ solid solutions on cooling and heating, even at low
($x$<0.10) tungsten content. It was found that tungsten ions have octahedral
coordination for in the whole studied temperature range (90-420 K), whereas
their coordination changes from tetrahedral to octahedral upon cooling for
smaller ($x$ <= 0.15) tungsten content. Nevertheless, some amount of tungsten
ions was found to co-exists in the octahedral environment at room temperature
for $x$<0.15. The obtained results correlate well with the color change in
these solid solutions.
