Type transformation in CuInSe2 and CuInS2 solar cells is an important issue with far reaching consequences. In the present study, the presence of a p-n homojunction inside CuInS2 in a TiO2/CuInS2 device is revealed with a detailed impedance spectroscopy and capacitance study. A n-type CuInS2 film with a thickness of 40?nm is found at the TiO2 (n-type)/CuInS2 (p-type) interface. The effective donor density of this n-type film is 2×1017?cm?3 at 400?K and is higher than the effective acceptor density in the remaining p-type CuInS2, being 4×1016?cm?3 at 400?K. Both densities decrease upon increasing the temperature. This is explained by the activation of a CuIn? acceptor state in n-type CuInS2 and a thermally activated hole trap in p-type CuInS2.