A technique is presented for the measurement of the absorption properties of liquids in the terahertz wave range, based on the interaction between the liquid and the evanescent wave covering the surface of a cylindrical waveguide made of high-resistivity silicon. The terahertz wave propagates inside the waveguide as the fundamental EH11 mode. The presence of a medium around the silicon rod has a measurable effect on the overall end-to-end transmission of the waveguide. As demonstration, we report the measurements of a D-glucose aqueous solution with a concentration ranging from 0% to 45%, using a backward-wave oscillator as the terahertz wave source. (C) 2008 American Institute of Physics.