A new clathrate solid solution BaCu − AuP (x = 4, 8, 12) was synthesized by a high-temperature solid-state annealing method. The crystal structures of BaCu − AuP were determined by single crystal x-ray diffraction. This clathrate solid solution crystallizes in the orthorhombic superstructure of clathrate-I type with 23 crystallographically independent framework sites, eight of them are occupied by Au/Cu and 15 are exclusively occupied by P atoms. The distribution of Au and Cu atoms over these eight framework sites is not random with a clear preference for Au to occupy the largest (Au/Cu)P tetrahedra in the framework. The thermal stability and thermoelectric properties of the BaCu − AuP solid solution were evaluated. Low thermal conductivity was achieved for BaCu − AuP due to the combination of the host–guest crystal structure with rattling Ba atoms with the presence of heavy Au atoms and substitutional Cu/Au disorder in the clathrate framework.