This study was concluded to elucidate a pathway for formation of C-19 isoprenoid hydrocarbons (isops) in petroleum from chlorophylls. C-19 isops are predominantly produced when dihydrophytol is heated at 320 degrees C for a period ranging from 1 to 5 h under vacuum while C-20 isops are predominantly produced when chlorophyll a or phytol is heated. A radical chain reaction of decomposition of dihydrophytol is proposed as plausible pathway for producing C-19 isops.