Elements with variable valence state (i.e. redox-sensitive) often show contrasting mineral/melt partition coefficients as a function of oxygen fugacity (fO2) in magmatic systems. This is because trace-element incorporation into crystal lattices depends on the charge, size, and crystal-field stabilization energy of atoms, all of which differ greatly between oxidized and reduced species of the same element. This has two critical implications: (1) petrologic/ geochemical modelling of partitioning behavior of redox-sensitive trace-elements in magmatic systems requires some knowledge of their oxidation state, and (2) the oxidation state of magmatic systems may be inferred from partitioning relations of redox-sensitive trace elements preserved in mineral and melt phases of rapidly cooled magmas. The advantage of this oxybarometric approach is that mineral/melt partitioning relations are not sensitive to late stage degassing, charge-transfer on quenching, or surficial alteration. In this chapter we discuss the theoretical treatment of experimental mineral/melt partitioning data of redox-sensitive trace elements, and review aspects concerning the partitioning behavior of well-known redox-sensitive elements, including transition metals (Ti, V, Cr, Fe), rare earth elements (Ce, Eu), U, and siderophile elements (Mo, W, Re, and platinum group elements) under planetary magmatic fO2 conditions.