Infrared spectroscopic characterization of the low-albedo materials on Iapetus

► We analyze the spectral signature of the dark materials on Iapetus. ► We find two distinct kinds of hydrocarbon bands in a unique combination. ► The hydrocarbon signature on the leading side of Iapetus matches Phoebe's. ► The hydrocarbon signature on the trailing side of Iapetus matches Hyperion's.