Herein, we report the liquid properties, that is, densities, viscosities, electrical conductivities, surface tensions, melting points, and glass transition temperatures, of 18 novel sulfur-containing phosphonium cation- (triethyl[(methylthio)methyl]phosphonium, triethyl[2-(methylthio)ethyl]phosphonium, triethyl[2-(ethylthio)ethyl]phosphonium, tributyl[(methylthio)methyl]phosphonium, tributyl[2-(methylthio)ethyl]phosphonium, tributyl[2-(ethylthio)ethyl]-phosphonium) based ionic liquids with bis(trifluoromethylsulfonyl)amide, bis(fluorosulfonyl)amide, and tetrafluoroborate anions. The effects of the (alkylthio)-alkyl groups on the viscosity and surface tension of the ionic liquids are very different from those of common alkyl groups in typical ionic liquids.