The band gap engineering of semiconducting oxide to shift the light absorption edge to lower energies in the visible spectrum is often achieved by doping the bulk material. Intercalation of atomic and small molecular species in the cavities of WO3 is a viable approach to introduce foreign elements in the lattice (Mi, Q.; Ping, Y.; Li, Y.; Cao, B.; Brunschwig, B. S.; Khalifah, P. G.; Galli, G. A.; Gray, H. B.; Lewis, N. S.; J. Am. Chem. Soc. 2012, 134, 18318). In this work we show that introduction of alkali metal cations in room-temperature (RT) monoclinic WO3 interstices causes an anisotropic distortion of the lattice parameters (b/c ratio is smaller than that of the pure bulk value) which is the major cause of a considerable band gap reduction (about 0.5 eV). Analogous intercalation of neutral atoms (i.e., Ne and Xe) induces only a tiny b/c ratio variation in the opposite direction with no effective band gap reduction. Structural differences in WO3 (e.g., from RT monoclinic to cubic phase WO3) are known to largely change the band gap value. We show that the lattice distortions caused by small amounts of interstitial alkali metal atoms, although not large, induce considerable variations in the band gap. © 2014 American Chemical Society.