Investigations on pure superconducting phase TlBa2Ca2Cu3Ox (Tl-1223) thin films formation, of about 100-125 nm in thickness, on (001) LaAlO3 single crystal substrate, were made using radio-frequency sputtering deposition of Ba2Ca2Cu3Ox x precursor films and ex-situ thallination in sealed quartz tube. The precursor films were thallinated under different conditions of partial oxygen pressure, temperature, time and y thallium source content using unreacted pellets of composition TlyBa2Ca2Cu3Ox. In all cases, strongly c-oriented multiphase films were obtained. A correlation between the Tl-1223 phase purity and the precursor film conditions of thallination is established. Temperature and time of thallination as well as the thallium source content and the partial pressure of oxygen play a key role in the quality of the obtained film. The films' onset temperature of the superconducting transition ranges between 90 and 103 K. It is shown that the best samples can be obtained from a dense precursor film and relatively medium thallination time. (C) 2010 Elsevier B.V. All rights reserved.