The preparation of Tl2Ba2Ca2Cu3Oy thick films from spin-coated Ba-Ca-Cu-O films in the presence of thallium oxide vapor was studied. The starting Ba-Ca-Cu-O films were prepared by pyrolysis of organic acid salts. The preparation of the Tl2Ba2Ca2Cu3Oy thick films was carried out for cases of the presence of Tl2O3 powder and of mixed powders of Tl2O3, BaO2, CaO2 and CuO. A Tl2Ba2Ca2Cu3Oy film with Tc onset=126 K, Tc mid=115 K and Tc zero=108 K was obtained. Control of the thallium content of the films was more easily accomplished when using the mixed powders of Tl2O3, BaO2, CaO2 and CuO than when using Tl2O3 powder alone.