Superconducting Tl2Ba2CaCu2Ox, (2212) films have been prepared by annealing the precursor Ba-Ca-Cu-O thin layers in an atmosphere containing vapours of thallous oxide. The precursor films were deposited from aqueous aerosol of the corresponding nitrates onto MgO substrates. As a source of Tl2O thallination pellets prepared by homogenizing of a mixture of Ba-Ca-Cu-O precursors with Tl2O3 were used. After common annealing of the film and the thallination pellet, which were in close contact, superconducting (2212) phase was obtained in the him as well in the pellet. The best T-c values of the synthesized (2212) films and pellets were 103.5 and 102 K, respectively. The chemical aspects of both thermal decomposition reactions during film growing as well as those proceeding during the thallination process, are discussed from the point of view of thermogravimetric TG, DTG and DTA measurements.