In pursuit of the elusive polycyclic arene episulfides, the synthesis of phenanthrene-9,10episulfide (Sb) was attempted. However, the reactions of phenanthrene-9,10-oxide and of phenanthrene with sulfur transfer agents and of 5,7-dihydrodibenzo[c,e]thiepine (9) with butyllithium did not afford fib, although its intermediacy could be detected by NMR and inferred from the products isolated. Quanturn-mechanieal ab initio and density functional methods were used to calculate the thermodynamic stability of representative arene episulfides and arene oxides. The arene episulfides were found to be thermodynamically significantly less stable than the corresponding oxides, towards elimination of sulfur and oxygen respectively. K-region arene episulfides are found to be thermodynamically more stable towards sulfur extrusion than their bayregion isomers. The thermodynamic stabilities of analogous arene oxides and arene episulfi~es parallel each other, and the relative thermodynamic stabilities of arene oxides and episulfides can, be deduced from the degree of the aromatic character of the rings which do not carry the heteroatomi