Intermetallic type-I clathrates continue to attract attention as promising thermoelectric materials. Here we present structural and thermoelectric properties of single crystalline Ba 8 (Cu,Ga,Ge, ) 46 , where denotes a vacancy. By single crystal X-ray diffraction on crystals without Ga we find clear evidence for the presence of vacancies at the 6c site in the structure. With increasing Ga content, vacancies are successively filled. This increases the charge carrier mobility strongly, even within a small range of Ga substitution, leading to reduced electrical resistivity and enhanced thermoelectric performance. The largest figure of merit ZT = 0.9 at 900 K is found for a single crystal of approximate composition Ba 8 Cu 4.6 Ga 1.0 Ge 40.4 . This value, that may further increase at higher temperatures, is one of the largest to date found in transition metal element-based clathrates. Vacancy filling by Ga substitution enhances the TE performance of type-I clathrates via improving charge carrier mobility.