Partial substitution of oxygen by fluorine in TlBa2CuO5 has been achieved by heating Tl2O3, BaO2, BaO and CuO under one kbar external pressure using TlF as a source for fluorine. A single phase region exists for 0.1 less-than-or-equal-to x less-than-or-equal-to 0.5 and the X-ray powder diffraction data indicated a tetragonal structure (space group : P4/mmm) for all the compounds. The in-plane Cu-0 distance calculated from the a-lattice parameter showed a systematic increase as x was varied from 0.1 to 0.5. This is due to an increase in the formal oxidation state of copper as F1- substitutes for O2- in the 1201-type structure. The c-lattice parameter also showed a slight increase, as the fluorine content in the reaction mixture is increased. Superconductivity is observed for x values within the range 0.1 to 0.6 with transition temperatures in the range 35-75 K.