H-transfers for upgrading of bridge polyaromatic structures in coal liquids and heavy oil fractions were studied in tetralin (THN) as H-donor model system. Symmetrical hydrogenation of t-stilbene (PhiCH=CHPhi), in the presence of diphenyidisulfide (PhiSSPhi), and scission of bibenzyl (BB), as representative of consecutive stages of a same process, were investigated. The results have shown that (1) reduction of t-stilbene to BB, at 350 degreesC, occurs with stoichiometric, yield (100%) when accelerated by addition of diphenyldisulfide as radical generator; (2) scission of BB, at 360-422 degreesC, takes place for 99.7 to 95.4% according to an unsymmetrical mechanism (formation of benzene) due to H-ipso attack from THN to stronger C-ar-C-al bonds and for the remaining 0.3 to 4.6% with a symmetrical mechanism (formation of toluene) due to weaker C-al-C-al bond homolysis. (C) 2002 Elsevier Science B.V All rights reserved.