The unstable acetonitrile N-oxide molecule, CH3CNO, has been thermolytically generated in very high yield in the gas phase from its stable ring dimer, dimethylfuroxan, and studied by ultraviolet photoelectron spectroscopy, photoionization mass spectroscopy, and mid-infrared spectroscopy. The individual spectroscopies provide a detailed investigation into the vibrational and electronic character of the molecule, and are supported by both conventional ab initio calculations and density functional theory. The ground-state structure is also investigated by theory at the B3-LYP, MPn (n = 2-4), QCISD, and QCISD(T) levels with medium to large basis sets, and illustrates the need for a precise description of electron correlation. Given that both isomerization and dimerization are feasible loss processes for this unstable molecule, the relative stability of CH3CNO with respect to the known cyanate (CH3OCN), isocyanate (CH3NCO), and fulminate (CH3ONC) isomers and the mechanism of the dimerization processes were studied with density functional theory.