Bulk Y1Ba2Cu3O7-δ superconductors were prepared from initial mixtures of a precursor and CsI or commercial 123 Hoechst powder and CsI with the nominal composition Y1Ba2Cu3CsxOy (x = 0, 0.05, 0.1, 0.2 and 0.3). Different synthesis regimes were applied in order to determine the influence of normal or reduced oxygen pressure as well as vacuum when isothermal heating at 800°C is performed during sintering. It was found that heating at 800°C with reduced oxygen pressure favourably affects the Jc and density of the samples in the case where 123 powder is used as the starting material. When the precursor is used the intensive dissociation of BaCO3 at low pressure leads to higher porosity and deterioration of the superconducting properties. Adding Cs in small concentrations promotes higher Jc and density of specimens made from the precursor, but when commercial 123 powder is used, a negative effect is observed due to the chemical instability of the superconducting phase. Atomic absorption spectrometry reveals that the Cs disappears during sintering and, therefore, could be used in small concentrations as a fluxing additive without any risk of creating Cs-containing impurity phases. © 1996 Chapman & Hall.