[en] We report the carrier transport properties of CuIn(S_1-xSe_x)₂ (0 ≤ x ≤ 1), a promising chalcopyrite semiconductor series for solar water splitting. A low concentration Mg dopant is used to decrease the carrier resistivity through facilitating bulk p-type transport at ambient temperature. Temperature-dependent resistivity measurements reveal a four-order magnitude decrease in bulk electrical resistivity (from 10³ to 10^–1 Ohm cm) for 1% Mg-doped CuIn(S_1–xSe_x)₂ as x increases from 0 to 1. Hall effect measurements at room temperature reveal p-type majority carrier concentrations that vary from 10¹⁵ to 10¹⁸ cm^–3 and mobilities of approximately 1–10 cm² V^–1 s^–1. These results provide insights into the fundamental carrier transport properties of CuIn(S_1–xSe_x)₂ and will be of value in optimizing these materials further for photoelectrochemistry applications.