A comprehensive study on the reactivity patterns and reaction mechanisms in alkane hydroxylation, olefin epoxidation, cyclohexene oxidation, and sulfoxidation reactions by a mononuclear nonheme ruthenium(IV)-oxo complex, [Ru(IV)(O)(terpy)(bpm)](2+) (1), has been conducted experimentally and theoretically. In alkane hydroxylation (i.e., oxygen rebound versus oxygen non-rebound mechanisms), both the experimental and theoretical results show that the substrate radical formed via a rate-determining hydrogen atom (H-atom) abstraction of alkanes by 1 prefers dissociation over oxygen rebound and desaturation processes. In the oxidation of olefins by 1, the observations of the kinetic isotope effect (KIE) value of 1 and the styrene oxide formation lead us to conclude that an epoxidation reaction via an oxygen atom transfer (OAT) from the Ru(IV)O complex to the C=C double bond is the dominant pathway. DFT calculations show that the epoxidation reaction is a two-step two-spin state process. In contrast, the oxidation of cyclohexene by 1 affords products derived from the allylic C-H bond oxidation, with a high KIE value of 38(3). The preference of the H-atom abstraction over the C=C double bond epoxidation in the oxidation of cyclohexene by 1 is elucidated by DFT calculations, in which the C-H activation energy barrier is 4.5 kcal mol(-1) lower than the epoxidation energy barrier. In the oxidation of sulfides, the sulfoxidation by the electrophilic Ru-oxo group of 1 occurs via a direct OAT mechanism, and DFT calculations show that this is a two-spin state reaction where the transition state is the lowest in the S = 0 state.