We have performed ⁶³Cu, ⁶⁵Cu, and ¹³⁷Ba NMR on Ba₈Cu₅Si_xGe_41−x, a series of intermetallic clathrates known for their potential as thermoelectric materials, in order to investigate the electronic behavior of the samples. The spectra and spin–lattice relaxation times were measured at 77 K and 290 K for the entire composition range 0 ≤ x ≤ 41. Magnetic and quadrupole shifts and relaxation rates of the Cu NMR data were extracted, and thereby carrier-induced metallic contributions identified. The observed shifts change in a nonlinear way with increasing Si substitution: from x = 0 to about 20 the shifts are essentially constant, while approaching x = 41 they increase rapidly. At the same time, Ba NMR data indicate greater Ba-site participation in the conduction band in Ba₈Cu₅Si₄₁ than in Ba₈Cu₅Ge₄₁. The results indicate surprisingly little change in electronic features vs. Si content for most of the composition range, while Ba₈Cu₅Si₄₁ exhibits enhanced hybridization and a more metallic framework than Ba₈Cu₅Ge₄₁.