Aromatic hydrocarbons (biphenyl, naphthalene, anthracene and phenanthrene) were subjected to ball milling (SPEX 8000) with approximately ten-fold weight of inorganic materials (alumina or silica). After about 24 h ail of the hydrocarbons were converted largely to carbon (graphite), but at intermediate stages disproportionation products (tetralin, phenylcyclohexane, bicyclohexyl, 9,10-dihydroanthracene, 1,2,3,4-tetrahydroanthracene, 1,2,3,4,4a,9,9a,10-octahydroanthracene, 1,2,3,4,5,6,7,8-actahydroanthracene, 9,10-dihydrophenanthrene, 1,2,3,4-tetrahydrophenanthrene, 1,2,3,4,4a,9,9a,10-octahydrophenanthrene, 1,2,3,4,5,6,7,8-octahydrophenanthrene) were also obtained in significant yields.