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The study of photoelectron satellites in the near-threshold region is important for differentiating among various electron correlation mechanisms. For CO, which is a prominent test case for theories describing electron correlation effects, the threshold and near-threshold branching ratios (BR s) of the K-shell satellites were measured. Eight different C 1s satellite states were identified, showing very complex threshold behavior. The resonancelike behavior of the {20. ) '(1n. ) '(2~)'(5'=1) satellite BR agrees with recent theoretical calculations.The observed energy dependence of the singlet (2o. ) '(1m) '(2m. *)' satellite compares less well with the calculations. Three X+ satellites with a (2o. ) '(50 ) '(no. )' configuration have a constant satellite BR over the energy range studied. Three 20.satellites, which have been assigned as pure conjugate shake-up states, have intensity only near threshold. The threshold spectrum above the 0 1s ionization threshold consists of two distinct groups of satellites. Conjugate shake-up transitions contribute to the intense feature centered at 551-eV binding energy, which is not observed above its threshold. Two 'X+ satellites, which have been described by configuration interaction of (1') '(50. ) '(no. )' states with a singlet mm. * state, show indications of shape-resonance-like behavior.PACS number(s): 33.60. Fy, 33.80.b