BiCuOS, which is isostructural to the layered rare-earth oxysulfides LnCuOS (Ln = La-Eu), was synthesized by a single-step hydrothermal reaction at low temperature (250 degrees C) and pressure (<20 atm). Particular emphasis is placed on how the selection of the proper reaction conditions, such as temperature and pH, achieves a mutual high solubility of the metal-oxide reactants, Bi2O3 and Cu2O, and thus generates BiCuOS in a good yield. The optical and electrical properties of BiCuOS were measured to determine the influence of replacing a rare-earth cation with bismuth. The electrical conductivity of BiCuOS is increased over that of certain layered rare-earth oxysulfides, LnCuOS (Ln = La, Pr, and Nd), and is similar to that of the cerium members, CeCuOS and CeAgOS. Band structure calculations reveal that, similar to other potential transparent conductors containing sixth-row elements, relativistic effects significantly lower the energy of the conduction band, and thus narrow the optical band gap. These low-energy conduction bands are responsible for the electrical and optical properties of BiCuOS.