Rotationally cold absorption and two-photon ionization spectra of CO in the 90-100 nm region have been recorded at a resolution of 0.3-1.0 cm-1. The analyses of up to four isotopomers seek to clarify the observations in regions where the Rydberg levels built on the ground state X ²Sigma⁺ of the ion interact with valence states of ¹Sigma⁺ and ¹Pi symmetry. Previous observations of the 3ssigma, B ¹Sigma⁺ Rydberg state, reviewed by Tchang-Brillet et al. [J. Chem. Phys. 96, 6735 (1992)], have been extended to energies above its avoided crossing with the repulsive part of the D^{' 1}Sigma⁺ valence state where resonances of varying intensities and widths have been attributed to the fully coupled 3ssigma or 4ssigma and D^' potentials, and where the B state approaches a second avoided crossing with the C^{' 1}Sigma⁺ valence state [Cooper and Kirby, J. Chem. Phys. 87, 424 (1987); 90, 4895 (1989); Chem. Phys. Lett. 152, 393 (1988)]. Fragments of a progression of weak and mostly diffuse bands, observed for all four isotopomers, have been assigned to the C^'