The present study was conducted to assess the structures of the main unknown oxygenated metabolites of EAPB0203. The first step was to assign all the (1)H and (13)C NMR of both EAPB0203 and its demethylated metabolite (EAPB0202) to the corresponding atoms in their molecular structures and to elucidate the fragmentation pathways for the [M + H](+) ions of these compounds using high-resolution mass spectrometry (MS). MS/MS spectra showed that both protonated molecules possessing an even number of electrons were unexpectedly losing radicals such as H(•), CH(3)(•), or even C(7)H(7)(•) giving stable radical cations. In vitro metabolism studies were investigated in rat and dog liver microsomes and in the filamentous fungus Cunninghamella elegans. Structural elucidation of six oxygenated metabolites was performed based on the following: (i) their fragmentation pathways in liquid chromatography-MS/MS (LC-MS/MS) analyses; (ii) comparison of their changes in their molecular masses and fragment ions with those of the parent drugs; and (iii) the results of online H/D exchange experiments that provided additional evidence in differentiating hydoxylated metabolites from N-oxides. Structures of the metabolites were elucidated by LC-MS/MS and comparison with synthetic standards; structures of these standards were confirmed using one- and two-dimensional (1)H NMR spectroscopies.