The reaction of ergosterol (ERGO) with singlet oxygen in vitro was studied by using different combinations of the photosensitizers (i.e. rose Bengal and eosine) and solvents (i.e. pyridine, ethanol and methyl tert-butyl ether) and all the products obtained were isolated and fully characterized (mp, Rf, UV, 1H-NMR, 13C-NMR, EI-MS, ESI-MS and HR-MS). In pyridine. together with the expected (22E)-5alpha,8alpha-epidioxyergosta-6,22-dien-3beta-ol, EEP, the keto derivative (22E)-3beta-hydroxyergosta-5,8(9),22-trien-7-one. KE, was obtained. In ethanol the expected EEP was obtained together with (22E)-5alpha,8alpha-epidioxyergosta-6,9,22-trien-3beta-ol, EEP9(11), and (22E)-ergosta-6,9,22-triene-3beta,5alpha,8alpha-triol, DHOE, as main products and (22E)-ergosta-5,7,9,22-tetraen-3beta-ol, DHE, in trace amounts In methyl tert-butyl ether, a complex mixture of EEP, KE, DHOE, EEP9(11), DHE, together with (22E)-7alpha-hydroperoxyergosta-5.8(9),22-trien-3beta-ol, EHP, and (22E)-ergosta-5,8(9),22-triene-3beta, 7alpha-diol, EH, was obtained. The minor products were characterized and showed strong dependence on the reaction medium. The regioselective and stereoselective character of the singlet oxygen attack on the ERGO diene moiety is discussed in terms of ERGO HOMO's properties.