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In principle, thermoelectric materials with superior performance require high lattice symmetry and heavy components integrated in semiconductors. Cu 6 WSnSe 8 , isostructural to kiddcreekite Cu 6 WSnS 8 , possesses the characteristics for being a promising thermoelectric material, including narrow band-gap, high crystal symmetry, heavy constituent elements, and complex crystal structure. In this work, the thermoelectric transport properties of pristine and copper-deficient Cu 6 WSnSe 8 were investigated. The extremely low hole mobility restricts Cu 6 WSnSe 8 as an excellent thermoelectric parent despite the fact that Cu-deficiency can increase the electrical conductivity and boost the power factor of Cu 5.94 WSnSe 8 by four times than that of pristine Cu 6 WSnSe 8. Combined with density functional theory calculations and X-ray photoelectron spectra analysis, we conclude that the extremely low hole mobility derives from the charge localization around W cations. Due to charge transfer and redistribution, the high valence state W6+ seizes electrons from Se2−, and thus localizing carriers. [Display omitted] • Kiddcreekite-type Cu 6 WSnSe 8 has been synthesized by mild solid-state reaction method at 773 K. • Cu-deficiency can significantly increase the electrical conductivity and boost the power factor. • The presence of W blocks the hole transport and results in ultralow hole mobility. [ABSTRACT FROM AUTHOR]