The thermolysis of 1,2-di(1-naphthyl)ethane (DNE) under pressurized hydrogen or nitrogen with or without a hydrogen donor additive in decalin was investigated to examine the effects of molecular hydrogen and several hydrogen donor additives on DNE thermolysis. The results show that molecular hydrogen and hydrogen donors play very different roles in DNE thermolysis: molecular hydrogen promotes DNE thermolysis, whereas the addition of a hydrogen donor inhibits DNE thermolysis.