We report the frequency-dependent optical constants, n(nu) and alpha(nu), or, equivalently, the complex permittivity <(epsilon)over cap>(omega) = epsilon’(omega) - i epsilon "(omega), over the frequency range from 2 to 50 cm(-1) for water, methanol ethanol, 1-propanol, and liquid ammonia. These spectra have been measured with femtosecond terahertz pulse transmission spectroscopy. These liquids exhibit multiple-Debye behavior, making their frequency-dependent dielectric constants valuable benchmarks for molecular dynamics simulations and other theoretical treatments of liquids.