Superconducting thin films of Bi-Sr-Ca-Cu-O have been prepared on (100) magnesium oxide substrates using metal trifluoroacetate spin-on precursors. The superconducting phase was formed by a three step process: (a) decomposition of the spun-on trifluoroacetate film to the fluorides, (b) conversion of the fluorides to oxides by reacting with water vapor, and (c) annealing in oxygen at 865-890°C. The film properties were found to be sensitive to the final annealing conditions. Films annealed at temperatures around 865°C in O2 formed almost exclusively the low temperature Bi2Sr2Ca1Cu2Ox phase, with strong c-axis normal preferred orientation and showed zero resistance around 77 K. When the films were annealed close to the melting point (∼885 K), a significant drop in resistivity was observed around 110 K, with zero resistance achieved at 77-80 K. X-ray diffraction results showed primarily the formation of the low temperature phase and only a small fraction of the high temperature Bi2Sr2Ca2Cu3Oy phase. Attempts to stabilize the high temperature phase by partial substitution of lead for bismuth were also not successful under these annealing conditions. © 1989.