X-ray absorption fine structure spectroscopy at the Ga and Ge K-edges was used to study changes in the Ga and Ge electronic structure and local coordination geometry as a function of composition in Ba(8)Ga(16)Ge(30-x)Si(x) type I clathrates (x = 0, 7.5, 9.1, 10.7, 13.4 and 30). Based on XANES data, the partial density of unoccupied states with p character is modified for both Ga and Ge upon Si substitution. Among the specimens which contain both Ge and Si, we found that the specimen with the highest measured power factor (i.e., x = 7.5) has the lowest density of unoccupied states for both Ga and Ge. Our experimental results are qualitatively consistent with computational results based on density functional theory, indicating that a series of pertinent electronic states are modified by Si p states. This suggests that an increase in the electron density near the Fermi level for an optimal Si substitution leads to an increase in the Seebeck coefficient and consequently in the power factor, according to the Cutler-Mott relation. Based on quantitative analysis of EXAFS spectra, we found that Ga has more Si neighbors than Ge, indicating that Si resides preferentially next to Ga. Both the Ge-Ga/Ge and Ga-Ge/Ga coordination distances remain relatively unchanged (~2.51 Å) regardless of the degree of Si substitution. Furthermore, the Ge-Si and Ga-Si coordination distances remain relatively unchanged at ~2.41 and ~2.45 Å, respectively, regardless of the degree of Si substitution. For the Ba(8)Ga(16)Si(30) specimen, on average, Ga is coordinated with 0.9 Ga and 3.1 Si at roughly the same distance of ~2.50 Å. The number of Ga-Ga bonds is consistent with the fact that Ga is distributed on the framework sites in a way which reduces the number of Ga-Ga bonds relative to that based on a random distribution. An understanding of the underlying physics of the structure-property relationship provides for potential additional routes for tuning the electronic properties of clathrates for thermoelectric applications.