The complete-active-space self-consistent field (CASSCF) and density functional theory (DFT) approaches have been used to study the mechanistic details of Norrish type I and II reactions of aromatic carbonyl compounds, with butyrophenone (PhCOCH2CH2CH3) as a representative. A minimum energy crossing point was found to exist among three potential energy surfaces (S-1, T-1, and T-2), and the three-surface crossing allows the T-1 state to act as a relay that enables the intersystem crossing (ISC) from S-1 to T-1 to occur with a high efficiency for PhCOCH2CH2CH3. Once the molecule is in the T-1 state, the 1,5-H shift reaction is the predominant reaction pathway and yields a triplet 1,4-biradical of PhC(OH)CHCH,CH? as an intermediate species. Since the formation of excited triplet products is energetically improbable, the subsequent decomposition, cyclicization, and disproportionation of the 1,4-biradical proceed after intersystem crossing from the triplet to singlet state. The singlet 1,4-biradical was found to have three isomers, which determine to a certain extent the branching ratios of the subsequent reactions. The study given here provides new insights into the S-1 relaxation dynamics of aromatic carbonyl compounds and their subsequent reaction mechanisms.