The chemical stability of (Tl,Pb)(Sr,Ba)2Ca2Cu3Oy, single crystals was studied in environments such as solutions of inorganic acids, air, water and organic solvents. Optical microscopy was used to check the surface changes of the crystals and the colors of the reaction products in the solutions or of the particles formed on the sample surface. The microscopic morphologies of reacted surfaces and the elements contained in the reaction products were investigated by scanning electron microscopy and energy dispersive analysis, respectively. It was found that the stability of (Tl,Pb)(Sr,Ba)2Ca2Cu3Oy single crystals in water is better than that of the YBCO and BiSCCO superconductors. Their reaction with HCl and HNO3 is very quick, but slow with H2SO4 or HF. Possible corrosions of Tl-1223 crystals in organic solvents were also checked (methanol, ethanol, acetone solvents).