The phase diagram of solid carbon monoxide was determined by x-ray diffraction and Raman spectroscopy from 15 to 297 K at pressures from 1.0 to 10.0 GPa. At low temperature a transition occurs near 3.4 GPa from the known α-phase (space group P2l3) to a new ε-phase (structure unknown), rather than to the predicted γ-phase (P42/mnm). A transformation from β-CO (space group P63/mmc) into a new δ-phase (Pm3n) was found near 5.2 GPa at room temperature. Above about 4.6 GPa and 80 K, CO reacts photochemically when irradiated with visible laser light. The photoreactivity may be associated with the formation of a yellow polymer, which can be recovered at zero pressure. ESCA measurements indicate that the polymer is composed of carbon suboxide (C3O2)x