Density, viscosity, specific conductivity, and freezing point measurements over the whole composition range of water–trifluoromethanesulfonic acid mixtures are reported. A maximum in viscosity and minimum in specific conductivity are observed at the composition corresponding to the monohydrate, H3O+SO3CF3−, and a maximum deviation from the ideal density is found at the composition corresponding to H5O2+SO3CF3−. The freezing point curve gives evidence for four hydrates with 1:1, 2:1, 4:1, and 6:1 water/trifluoromethanesulfonic acid mole ratios. An incongruent melting hydrate, probably 2:1 acid to water mole ratio is also indicated.