An influence of carbonate, sulphate- and berate ions on radiation resistance (RR) of CsI(Tl) crystals has been studied. The data demonstrate that berate ions affect the RR as well as other oxygen containing impurities (OI). Trace amounts of berate ions lead to colouring of the crystal under gamma rays, and decreasing of its light output. The method of purifying the melt of OI in the process of automated pulling of crystals by treating it with those high-melting metals, which generate insoluble oxides in the melt is suggested. Crystals, pulled out of the purified melt are highly resistant towards radiation and do not contain absorption bands in the IR spectrum, which is a distinctive feature of berate, carbonate and sulphate ions.