Eucalyptus tar pitches are generated on a large scale in Brazil as byproducts of the charcoal manufacturing industry. They have a macromolecular structure consisting mainly of phenolic, guaiacylic, and siringylic units substituted and interlinked by aliphatic side chains rich in oxygenated polar groups (such as alcohol, ether, carbonyl, and carboxyl), similar to lignin. Under treatment around 250degreesC, the pitches undergo polymerization. C-O links are broken up, and, while side chains are released, the heavier fragments rich in aromatic rings react to form larger molecules. The presence of AlCl3 and CH2O makes polymerization at lower temperatures (150degreesC and 100degreesC, respectively) possible. AlCl3 acts as a Lewis acid, complexing to oxygen atoms and weakening C-O bonds. In turn, CH2O acts as a polymerizing agent, as in the preparation of resol phenolic resins. These pretreatments have made it possible to adjust the softening point in order to yield spinnable pitches susceptible to post spinning thermal stabilization, besides yielding pitches with improved thermal stability. These possibilities are important because they make Eucalyptus tar pitches potential carbon fiber precursors. (C) 2003 Wiley Periodicals, Inc.