Single-phase samples of the ‘90 K’ BiSCCO superconductor can be prepared, by careful attention to the experimental conditions, for the compositions: (a) stoichiometric Bi₂Sr₂CaCu₂O_δ(b) Bi_2+X Sr_2–X CaCu₂O_δ 0 < X ⩽ 0.2; (c) Bi₂Sr_2–y Ca_1–y Cu₂ O_δ, 0 < y⩽ 0.5: (d) Bi_2+x Sr_2–x–y Ca_{1 +y} Cu₂O_δ, all of which retain the same (Bi + Sr + Ca) : Cu ratio of 5 : 2 as the ideal ‘2212’ Stoichiometry. The ranges of x and y values for the solid solutions are given in the form of a pseudoternary phase diagram. Lattice parameters were indexed on a tetragonal cell; a was independent of composition; c decreased with both x and y. Formation rates of the 90 K phase vary greatly with composition and generally increase with both x and y. The stoichiometric composition requires typically 5 days at 860 °C for complete reaction, whereas the composition x= 0, y= 0.5 requires only 2 days at 820 °C.