We examine the effect of alloying Sn on the Sb site of ytterbium-filled skutterudites, a promising class of thermoelectric materials. We report measurements of the Hall effect, electrical resistivity, Seebeck coefficient, and thermal conductivity between 2 and 300 K on two series of samples having different ytterbium filling fractions: Yb0.19Co4Sb12-xSnx, with x=0, 0.05, 0.1, and 0.2, and Yb0.5Co4Sb12-xSnx, with x=0.5, 0.6, 0.8, 0.83, and 0.9. We find that the substitution of Sn does not lower the electron concentration of these samples, but rather gives rise to a p-type carrier. Hall measurement data for Yb0.5Co4Sb11.17Sn0.83 can be understood in the context of two-carrier electrical conduction. Although the thermal conductivity of these ytterbium-filled skutterudites is significantly suppressed from that of the unfilled CoSb3, it is nearly independent of the Sn concentration. The role of phonon-point defect scattering and phonon resonance scattering in reducing the lattice thermal conductivity of the Yb-filled samples is assessed by a theoretical model.