The structures of benzofuroxan and o-dinitrosobenzene have been studied by semiempirical (AM1, PM3) and ab-initio methods (3-21G, D95, 6-31G, 6-31G*, MP2/D95//D95, MF2/6-31G//6-31G, MP2/6-31G*//6-31G*, MP2/6-31+G*//6-31G*). For o-dinitrosobenzene there exists a minimum of three different conformers (cis/trans, trans/trans, cis/anti). Whereas both semiempirical methods reveal a strong preference for the o-dinitrosobenzene isomer(s), the ab-initio calculations on the MP2 level result in a qualitatively correct prediction of experimental data.