Energy levels, specific mass shift parameters, hyperfine interaction constants, Landé gJ factors, and transition probabilities between computed levels are reported for C II, N III, and O IV. Results include levels belonging to 2s22p,2s2p2,2p3,2s23s,2s23p,2s23d,2s2p3s and, in the case of C II, the 2s24s and 2s24p configurations. Wavefunctions were determined using the multiconfiguration Dirac–Hartree–Fock method and account for valence, core–valence, and core–core correlation effects.