Coke deposition on hydrotreating catalysts is one of the main reasons of deactivation. In situ coke removal is advantageous compared to the off-site coke removal. This paper studies tetralin (THN) treatment of a deactivated industrial diesel hydrogenation Mo-Co-Ni/W-Al catalyst at conditions similar to the operation, 300-400 degrees C without H-2. The results show that the coke on the Mo sulfide sites, corresponding to 19-23% total coke, can be removed in 3 min to fully restore the catalyst's activity. The coke on the support cannot be removed. On the basis of the temperature-programmed oxidation, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron spin resonance, coke removal mechanism is discussed, which includes activation of H radicals in THN by the Mo sulfide sites and hydrogenation of coke by the H radicals. The minimal requirement for the coke removal is 340 degrees C for 3 min. The side reactions of THN are significant at higher temperatures and longer treatment times. The radical concentration of coke on the Mo sulfide sites is similar to that of lignites and is approximately 10 times the coke on the catalyst support.