Abstract This paper studies charge transfer processes in CsI:Tl crystals by analyzing the bulk photo-conductivity spectra, the temperature behavior of the bulk photo-conductivity current and the shape and intensity of the activator emission pulse excited by an electron pulse beam and/or laser pulse emission at temperatures between 80 and 400K. The Tl concentration in CsI:Tl crystals varies from 10−3–10−1mass%. It has been determined that near-UV light induces a bulk conductivity in CsI:Tl crystals only when the Tl concentration is greater than 3×10−3mass%. A mechanism is proposed to explain the charge transfer processes with photons whose energy is approximately half the width of the CsI band gap. Near-UV light causes charge transfer from I− to Tl+ ions, forming Tl0 centers in the 6p2P1/2 ground and 6p2P3/2 excited states. The electron, assisted by phonons, leaves the Tl0 center from either the 6p2P1/2 or 6p2P3/2 states and overcomes the 0.13 or 0.30eV energy barrier, respectively, and subsequently populates the activator conduction sub-bands, which are found inside the band gap of CsI:Tl. The formation of activator sub-bands is possible only above the threshold Tl concentration, i.e., above 3×10−3mass%.