Numerous (0- v″) bands of the CO A1Π- X1Σ + system have been photographed at high resolution in emission and absorption. The perturbation analysis of the A1Π, v = 0 level has been extended up to J = 75. Bands of intercombination systems, namely (1- v″) of the e3Σ -- X1Σ + and (4- v″) of the d3∆- X1Σ + bands, have been observed for J ≤ 40 because of intensity borrowing from the main A- X system. Above J ⋍ 40, perturbations arising from a' 3Σ +, v = 9; I1Σ -, v = 1; and e3Σ -, v = 2 are responsible for the irregular Λ-type doubling at the A1Π, v = 0 level. Accurate deperturbed constants for the A1Π state and the perturbing levels have been obtained; perturbation parameters and mixing coefficients which characterize each perturbation have been derived; and term values for the main and extra levels are presented.