By means of an electromagnetic driving and detection system investigations of mechanical energy losses under alternating stress have been performed on various carbon solids. Materials studied were a glassy carbon, various baked carbons and polycrystalline graphites, and some pyrolytic graphites. It was found that with decreasing strain amplitude the internal friction Q-i decreases and the dynamic elastic modulus E increases towards limiting values QO-' and Eo. Moreover, the amplitude dependent portion of the internal friction Q-i -QO-r is seen to be linearly proportional to the modulus defect (E,,)/E, for the same amplitude. A very weak neutron irradiation (dose less than 1Ol4 nvt) causes in some cases very large changes in the loss character. These facts can be explained by point imperfections pinning down dislocations. The temperature dependence of QO-' for polycrystalline graphite shows a so-called Bordoni peak at 50ยฐK for a frequency of about 12 kc accompanied by a small hump at 80ยฐK. The curve of E. against temperature shows steep drops in the same ranges of temperature. The Peierls potentials of the glissile dislocations in graphite corresponding to these two peaks are roughly estimated to he 0.07 and 0.12 eV respectively.