Monodimensional (chemical) diffusion measurements have been used to find a clean solid-state path to a pure Bi-Sr-Ca-Cu-O superconductor with two-layer structure, i.e., a suitable sequence of solid-state reactions such that each step gives a single-phase product. When starting from carbonate precursors of CaO and SrO plus Bi 2 O 3 and CuO, the critical point of a reaction path of this kind is an appropriate choice of the external conditions of temperature, oxygen partial pressure, and cation molecularities, which must be different from the ideal '2212' values. The results show that a clean solid-state synthesis of a single-phase two-layer material is achieved by reacting at 750 o C under P(O 2 ) = 10 - 3 atm the Sr 1 4 Cu 2 4 O 4 1 phase and the ternary solid solution with monoclinic symmetry and cation molecularity near 0.376:0.082:0.543 (Ca:Sr:Bi).