We carried out molecular orbital theory calculations for the homogeneous gas-phase formation of dibenzofuran from phenanthrene, fluorene, 9-methylfluorene and 9-fluorenone. Dibenzofuran will be formed if center dot OH adds to C-8a, and the order of reactivity follows as 9-fluorenone > 9-methylfluorene > fluorene > phenanthrene. The oxidations initiated by ClO center dot are more favorable processes, considering that the standard reaction Gibbs energies are at least 21.63 kcal/mol lower than those of the equivalent reactions initiated by center dot OH. The adding of center dot OH and then O-2 to phenanthrene is a more favorable route than adding center dot OH to C-8a of phenanthrene, when considering the greater reaction extent. The reaction channel from fluorene and O-2 to 9-fluorenone and H2O seems very important, not only because it contains only three elementary reactions, but because the standard reaction Gibbs energies are lower than -80.07 kcal/mol.