Cytochrome P450 (P450, CYP) enzymes are very versatile catalysts and, in the current literature, involved in more oxidations than any other family of enzymes. They oxidize drugs, steroids, vitamins, carcinogens, natural products, and industrial chemicals. Several of the 57 human P450s are critical for life, but others have been therapeutic targets. The chemistry of oxidation by these “mixed-function oxidases” involves reactions of heme-Fe²⁺ with O₂ and a series of unstable high-valent iron complexes, including Fe³⁺O₂^– (Compound 0), FeO³⁺ (Compound I), and FeOH³⁺ (Compound II). Discerning the roles of these individual species has been difficult–and controversial. At least the Compound I entity is capable of both hydrogen atom abstraction and coupled electron transfer. Recent studies in this laboratory have involved a series of isotopic labeling studies and analytical chemistry (mainly mass spectrometry) to distinguish roles of Compound I and Compound 0 in several reactions, particularly with carbonyls and several P450s. The balance between these paths is a reflection of substrate juxtaposition and the competition of proton transfer with other processes.