Thin films of cuprous iodide (CuI), which is expected to be used in solid-state dye-sensitized solar cells as a hole conductor, were formed by spin coating and subsequent annealing. The effects of the annealing conditions on the electrical properties of the CuI films were investigated by resistivity and Hall-effect measurements. The CuI films showed p-type conduction with resistivities of 0.3–5 Ω cm, carrier concentrations of 1 × 1018–1 × 1019 cm−3 and mobilities of 0.5–2 cm2 V−1 s−1. It was found that the resistivities of the films annealed in air were slightly lower than those annealed in an Ar atmosphere because the carrier concentrations were higher in the former than in the latter. An increase in the mobility with the rise of annealing temperature was observed in the films annealed in Ar. However, annealing in air at high temperatures oxidized CuI to CuO.