The structures and phase transitions of N2 and CO were studied by powder x-ray diffraction from 100 to 300 K and 4 to 13 GPa. Three solid phases, β, δ, and ε, were observed in each material. The known β and δ solids were confirmed to have hexagonal space group P63/mmc and cubic space group Pm3n, respectively. From refinements using photographic x-ray intensities, the new ε-N2 and ε-CO structures were determined to be rhombohedral R3̄c. There are eight ordered molecules in the ε-N2 unit cell with a=5.928 Å and α=85.14° at 110 K and 7.8 GPa, and eight ordered molecules in the ε-CO unit cell with a=6.059 Å and α=85.73° at 100 K and 5.5 GPa. The CO molecules are randomly oriented head to tail. The δ–ε transition takes place through an ordering and small displacement of the N2 and CO molecules, accompanied by a slight extension of the lattice along a cube diagonal. Molar volumes are presented over an expanded P-T region. Recent theoretical calculations using lattice energies, molecular dynamics, and symmetry correlations correctly predict features in the N2 and CO phase diagrams.