The synthesis and physical properties of the K(1-x)Ir4O8 (0 ≤ x ≤ 0.7) solid solution are reported. The structure of KIr4O8, solved with single-crystal X-ray diffraction at T = 110 K, is found to be tetragonal, space group I4/m, with a = 10.0492(3) Å and c = 3.14959(13) Å. A highly anisotropic displacement parameter is found for the potassium cation. Density functional theory calculations suggest that this anisotropy is due to a competition between atomic size and bond valence. KIr4O8 has a significant electronic contribution to the specific heat, γ = 13.9 mJ mol-Ir(-1) K(-2), indicating an effective carrier mass of m*/me ≈ 10. Further, there is a magnetic-field-dependent upturn in the specific heat at T < 3 K, suggestive of a magnetically sensitive phase transition below T < 1.8 K. Resistivity and magnetization measurements show that both end-members of the solid solution, KIr4O8 and K(1-x)Ir4O8 (x ≈ 0.7), are metallic, with no significant trends in the temperature-independent contributions to the magnetization. These results are interpreted and discussed in the context of the importance of the variability of the oxidation state of iridium. The differences in physical properties between members of the K(1-x)Ir4O8 (0 ≤ x ≤ 0.7) series are small and appear to be insensitive to the iridium oxidation state.