Significance In redox biology, the chemistry performed by proteins that contain the rare amino acid selenocysteine is frequently critical to the detoxification of reactive species that are harmful to cellular function. Selenocysteine and cysteine partner to form a motif featuring a sulfur–selenium covalent bond in many selenoproteins. This work demonstrates that selenium NMR, when paired with calculations, can provide critical insight concerning the local environment of these enigmatic redox motifs. It details how redox potentials, conformational preferences, and mobilities of such redox motifs change when the local environment of the selenocysteine is varied. Surprisingly, reverting selenocysteine to cysteine exerts only minor effects on redox potential. These new approaches deepen our understanding of the chemical reactivity and thermodynamic properties of selenoenzymes.