Abstract The effect of ion irradiation on the spectra of π-plasmon excitations requires close attention since the spectra are very sensitive to the degree of damage produced by ion bombardment of a sample under test. In this paper, the results of experiments on irradiation of multiwall carbon nanotube (MWNT) samples by argon ions with energy of 1 keV are presented. They were obtained by energy loss spectroscopy of reflected electrons and Auger electron spectroscopy. It was found that ion irradiation of MWNTs decreases the characteristic energy and increases the full-width at half-maximum of the π-plasmon peak. The presence of implanted Ar in the samples after the irradiation was determined by Auger electron spectroscopy. The Ar concentration varied from 1.1% to 5.4%. Modification of the KVV-line in Auger spectra indicates amorphisation of the carbon nanotubes surface.