The formation and decomposition of LaBa2Cu3O7-δ has been studied by X-ray diffraction and thermogravimetric analysis. The attempted preparation of LaBa2Cu3O7-δ in air or oxygen gives a mixture of a solid solution of composition La1+xBa2-xCu3O7-δ (x≈0.2), and BaCuO2 (J. M. S. Skakle and A. R. West, Physica C, 1994, 220,187). However, heating this mixture in a less oxidising atmosphere such as flowing argon at temperatures above 850 °C for a short time, ca. 16 h, yields a single-phase tetragonal product, which, when annealed in oxygen at lower temperature, gives an orthorhombic superconductor with Tc (onset) ≈ 94 K. The orthorhombic and tetragonal structures have been confirmed by Rietveld refinement of X-ray powder diffraction data; an orthorhombic-tetragonal transition occurs at δ = 0.2. The stability of the single-phase material under air and oxygen has been studied by thermogravimetry. It is stable up to 800 °C in air; at higher temperatures, decomposition to La1+xBa2-xCu3O7-δ (x≈0.2), and BaCuO2 occurs. On prolonged heating of single-phase tetragonal LaBa2Cu3O7-δ at high temperatures, e.g. 875 °C for 64 h under argon, decomposition to give La4-2xBa2+2xCu2-xO10-2* (x≈0.2), BaCuO2 and BaCu2O2 occurs. This is similar to the behaviour of YBCO under low oxygen pressure. Thus, under all conditions studied so far, LaBa2Cu3O7-δ may be considered to be a nonequilibrium phase. In orthorhombic samples of LaBa2Cu3O7-δ prepared by quenching, Tc varies linearly with δ; samples cooled slowly show evidence of a plateau at ca. 94 K for δ≈ - 0.05 to - 0.15.