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The thermal pyrolysis processes and solid-state reactions in the synthesis of bismuth-based high-Tc superconducting fibre obtained by using sodium alginates are studied. This alginate is a polyelectrolyte and has not only an excellent ion-exchange property but also a gelation to synthesize long fibrous precursors. We first determined the optimum conditions to promote ion-exchange of this polyelectrolyte which yields high contents of metal ions in the precursor. The thermal pyrolysis of a precursor was studied by DTA and IR absorption. On the basis of the DTA and XRD, the solid-state reactions associated with the formation of the 2 2 1 2 phase were revealed. Our method can be applied easily to synthesize Ag-sheathed fibre having an improved tensile strength.