Thallium bromide chloride (TlBr xCl 1-x) crystals have been evaluated as a material used for fabrication of room temperature radiation detectors. In this study, TlBr xCl 1-x crystals with various chlorine (Cl) concentrations were grown by the travelling molten zone method and the detectors were fabricated from the crystals. The optical properties of the crystals were evaluated by measuring the transmittances. The charge transport properties were characterized by the Hecht analysis. The band gap energy of the crystals proportionally increased with Cl concentration. Mobility-lifetime products (μτ) of the crystals decreased with increasing Cl concentration. © 2011 IEEE.