A new method for the identification of diffraction satellites accompanying surface plasmons (SPs) in the terahertz frequency range is proposed and verified. Reliable detection of these satellites allows the accuracy of plasmon spectrometers for the terahertz range to be significantly increased. The idea of the proposed method is based on the fact that a local thin-film coating element applied onto the SP-guiding surface in the absence of diffraction satellites must reduce the SP intensity by the same amount irrespective of the site of application. Experiments performed on the Novosibirsk free-electron laser facility confirmed the efficiency of the proposed method. © 2010 Pleiades Publishing, Ltd.