The complex refractive index of some organic liquids and solutions has been measured to 1 % or better by an optical method using as source a CW CN maser emitting at 29.7 cm–1(0.337 mm). Details of the measurements are given. The complex permittivity, calculated from the measured data, shows that most of the pure liquids have a loss consistent with the presence of a sub-millimetre absorption process while the complex permittivity for solutions of chlorobenzene in p-xylene appears to follow a simple mixture rule.