In X-ray irradiated KCl crystals containing more than 1 mole% of TlCl, ESR signals of Tl 2 3+ centers which are characterized by g =2.04 have been newly observed. This pair center has a 6 s hole whose wavefunction is almost localized on one of the thallium ions (hyperfine constant A =105 GHz), and only a little fraction (about 1%) of the wavefunction is distributed on the other thallium ion ( A '=1.0 GHz). Further investigations suggest that this trapped hole center is associated with a trapped electron center and exists in the form of (Tl 2 3+ –Tl 0 ). Another kind of associated center (Tl 2 + –Tl 2+ ) is also inferred to exist. The two kinds of associated centers are thermally more stable than any other trapped electron centers and trapped hole centers.