Infrared spectra of arsenic carbonyls trapped in an argon matrix by co-condensation of laser-ablated arsenic atoms and electrons with carbon monoxide are reported. Based on isotopic substitution experiments and theoretical calculations, an IR band at 1919.3 cm(-1) has been assigned to the C-O stretching vibration of the neutral AsCO, and a 1756.4 cm(-1) band to the C-O stretching mode of the AsCO- anion. The ground states of AsCO and AsCO- were predicted to be 2 Pi and (1)Sigma (+), respectively. The formation mechanism and the nature of bonding are discussed.