Analysis of HF/6-31G** and B3LYP/6-311G** relative energies of 25 unbranched catacondensed benzenoid hydrocarbons with six rings reveals a well-pronounced dependence of their thermodynamic stabilities on the presence of simple structural motifs. In particular, the energy contribution due to steric overcrowding is proportional to the number of pairs of adjacent angular annelations of the same helicity, whereas the contribution due to conjugation effects is related to the number and mutual position of linear annelations. Although the arrangement of linear annelations also uniquely determines the number of Kekule structures, the latter does not correlate directly with the non-steric energy component. (C) 2003 Elsevier Science B.V. All rights reserved.