The formation and decomposition of LaBa₂Cu₃O_{7 –δ} has been studied by X-ray diffraction and thermogravimetric analysis. The attempted preparation of LaBa₂Cu₃O_{7 –δ} in air or oxygen gives a mixture of a solid solution of composition La_{1 +x}Ba_{2 –x}Cu₃O_{7 –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 T_c(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 La_{1 +x}Ba_{2 –x}Cu₃O_{7 –δ}(x≈ 0.2), and BaCuO₂ occurs. On prolonged heating of single-phase tetragonal LaBa₂Cu₃O_{7 –δ} at high temperatures, e.g. 875 °C for 64 h under argon, decomposition to give La_{4 – 2x}Ba_{2 + 2x}Cu_{2 –x}O_{10 – 2x}(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, LaBa₂Cu₃O_{7 –δ} may be considered to be a non-equilibrium phase. In orthorhombic samples of LaBa₂Cu₃O_{7 –δ} prepared by quenching, T_cvaries linearly with δ samples cooled slowly show evidence of a plateau at ca. 94 K for δ≈–0.05 to –0.15.