This review discusses the thermochemistry of proton-coupled electron transfer (PCET) reagents and their implications in chemical and biochemical processes. PCET processes involve the simultaneous transfer of electrons and protons, which is crucial in many redox reactions, including those in biological systems and industrial processes. The review emphasizes the importance of understanding the thermochemistry of these processes, as it helps determine whether electron and proton transfers occur together in a single step or sequentially. The thermochemical data, including reduction potentials, acidity/basicity, and bond dissociation free energies, are essential for analyzing PCET mechanisms. The review also addresses the distinction between PCET and hydrogen atom transfer (HAT), highlighting the different mechanisms and implications of each. The thermochemical parameters, such as pKa values and redox potentials, are used to calculate bond dissociation free energies (BDFEs) and understand the energetics of PCET reactions. The review provides a comprehensive analysis of various PCET reagents, including hydroxylamines, phenols, and transition metal complexes, and discusses the importance of accurate thermochemical data in understanding and predicting PCET behavior. The review also addresses the challenges in determining BDFEs and the need for consistent experimental conditions to ensure accurate thermochemical data. Overall, the review underscores the significance of thermochemical data in elucidating the mechanisms and thermodynamics of PCET processes in both chemical and biological systems.This review discusses the thermochemistry of proton-coupled electron transfer (PCET) reagents and their implications in chemical and biochemical processes. PCET processes involve the simultaneous transfer of electrons and protons, which is crucial in many redox reactions, including those in biological systems and industrial processes. The review emphasizes the importance of understanding the thermochemistry of these processes, as it helps determine whether electron and proton transfers occur together in a single step or sequentially. The thermochemical data, including reduction potentials, acidity/basicity, and bond dissociation free energies, are essential for analyzing PCET mechanisms. The review also addresses the distinction between PCET and hydrogen atom transfer (HAT), highlighting the different mechanisms and implications of each. The thermochemical parameters, such as pKa values and redox potentials, are used to calculate bond dissociation free energies (BDFEs) and understand the energetics of PCET reactions. The review provides a comprehensive analysis of various PCET reagents, including hydroxylamines, phenols, and transition metal complexes, and discusses the importance of accurate thermochemical data in understanding and predicting PCET behavior. The review also addresses the challenges in determining BDFEs and the need for consistent experimental conditions to ensure accurate thermochemical data. Overall, the review underscores the significance of thermochemical data in elucidating the mechanisms and thermodynamics of PCET processes in both chemical and biological systems.