Abstract The type-I clathrates Ba8Cu5(Si,Ge,Sn)41 show promising thermoelectric (TE) properties. We investigated the influence of Sn substitution for Ge on the TE properties of Ba8Cu5Si6(Ge,Sn)35 samples prepared by the fast and economic melt–spinning technique and compared our results with a sample series prepared using the conventional synthesis method of high–frequency melting and hot pressing. All samples crystallize in cubic symmetry with the space group Pm 3 ¯ n. Scanning electron microscopy (SEM) together with energy dispersive x-ray spectroscopy (EDX) revealed a maximum solid solubility limit of 0.6 Sn atoms per unit cell. Electrical transport measurements showed metal–like behavior. The negative slope of the Hall resistivity ρH points on the dominance of electrons in this material, confirmed by Seebeck coefficient measurements. With increasing Sn content the Cu concentration decreases and vacancies appear. These secondary effects have a stronger influence on electrical transport than the direct influence of the Sn substitution. The electronic transport is dominated by alloy scattering at high temperatures and neutral–impurity scattering at low temperatures. The phononic thermal conductivity is by a factor of two lower than in the hot pressed samples. The dimensionless figure of merit ZT = 0.43 at 773 K is reached for the sample with the highest Sn content.