After deposition of laser ablated aluminum with CO in excess argon at 11 K, distinct IR bands at 1682.3, 1803.9, and 1741.0 cm(-1) have been observed in addition to the absorptions of neutral AlCO and Al(CO)(2). With the aid of (CO)-C-13 isotope enriched experiments and density functional calculations, the band at 1682.3 cm(-1) has been assigned to the C-O stretch vibration of the AlCO- anion. Absorptions at 1803.9 and 1741.0 cm(-1) have been assigned to the symmetric and antisymmetric C-O stretching vibrations of the Al(CO)(2)(-) anion, respectively. Vertical detachment energies of AlCO- and Al(CO)(2)(-) as well as the electron affinities of AlCO and Al(CO)(2) have been calculated. The properties of MCO- anions (M=B, Ga, and In) have also been predicted for comparison with those of AlCO-.