The (YBa,Cu,),-,Na,O,-, system in the range of x = 0 4 . 8 was investigated. Experimental data suggest that the sodium doping with x I 0.26 does not affect the critical transition temperature T,, and the crystal structure maintains the orthorhombic lattice with a slightly smaller unit cell. However, sodium doping increases the sintering and grain growth kinetics, resulting in a higher superconducting phase volume and an enhanced Meissner effect. It also lowers the processing temperature. The experimental data also suggest that the sodium atoms diffuse into the superconducting YBa,Cu,O,-, crystallites, which stabilizes the orthorhombic phase. The transition temperature (orthorhombic to tetragonal) in sodium-doped materials increases with the increasing concentration of sodium.