Single crystals of Ba8Au5.3Ge40.7 [space group Pm (3) over barn (No. 223), a = 10.79891(8) angstrom] were prepared by a Bridgman technique. The crystal structure refinement based on single-crystal X-ray diffraction data does not reveal any vacancies in the Au/Ge framework or in the cages. In addition to the ionic bonding between Ba and the anionic framework, a direct interaction between Ba and Au atoms was identified in Ba8Au5.3Ge40.7 by applying the electron localizability indicator. As expected by the chemical-bonding picture, Ba8Au5.3Ge40.7 is a diamagnet and shows p-type electrical conductivity with a hole carrier concentration of 7.14 x 10(19) cm(-3) at 300 K and very low lattice thermal conductivity of approximate to 0.6 W m(-1) K-1 at 500 K. The thermoelectric figure of merit ZT of single crystals of Ba8Au5.3Ge40.7 attains 0.3 at 511 K and reaches 0.9 at 680 K in a polycrystalline sample of closely similar composition. This opens up an opportunity for tuning of the thermoelectric properties of materials in the Ba-Au-Ge clathrate system by changing the chemical composition.