The AO + −X 1Σ + transition of TlCl is investigated by excitation spectroscopy and direct absorption detection with frequency-doubled laser radiation. From the resolved fine structure of all natural isotope combinations of TlCl, Dunham parameters, Born—Oppenheimer correction parameters and field-shift parameters are determined. The lower vibrational levels of AO + show complete rotational structure, while the upper levels v′ = 4, 5, 6 for the excitation spectra exhibit typical fragmentary bands as expected for predissociation where the rate of predissociation can become much larger than the rate of radiational decay. With direct absorption technique, these fragmentary bands are recorded as complete bands, containing broadened rotational transitions with half widths of more than 30 GHz. The qualitative interpretation of the predissociation is compared with the results of photofragment spectroscopy.