We have studied the effect of B2O3 addition on the formation and properties of the Tl-based superconductors. Polycrystalline Tl-2212 and Tl-2223 samples were synthesized by adding B2O3 to the chemicals at the first and at the second stage of two stages sintering processes (3+1 h) at 860 C. X-ray powder diffraction patterns, resistance versus temperature behaviours and the micrographs of the B2O3 free and B2O3 added samples have been analysed. The compositions and the critical temperatures of the samples indicate that about 1.0 weight percent B2O3 added at the first stage of sintering stabilize the Tl-2212 phase and at the second stage promote the formation of the Tl-2223 phase. For both of the phases, the a-lattice parameter remains the same, but the c-parameter slightly decreases with B2O3 additions. Few small peaks due to the impurity phases appear in the diffraction patterns but none due to B2O3 or any other phase involving B atoms. We suggest that, with their small size B atoms may partially substitute Cu atoms and disperse at interstitial positions and grain boundaries. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.