A modified synthesis for carbonyl diisocyanate, CO(NCO)(2), starting from trichloroisocyanuric acid and diphosgene is described. In addition to the previously reported C-13 NMR resonances, the N-15 NMR shift is determined for the first time. The structure in the solid state was determined by X-ray diffraction (XRD) on in situ grown crystals, that in the gas phase was experimentally determined by electron diffraction (GED) and for single molecules theoretically by quantum-chemical calculations. The structures are compared and discussed with related systems. Quantum-chemical calculations as well as GED and XRD prove syn-syn to be the conformation of lowest energy. In quantum-chemical calculations and GED the presence of a syn-anti conformer was confirmed and the structure of this conformer was determined.