We determined the phase stability limits of Bi2Sr2Ca1Cu2O8+delta and Bi2Sr2Ca2Cu3O10+delta in the temperature range 650-880-degrees-C using a solid-state electrochemical technique. These phases decompose by incongruent melting above approximately 790-degrees-C, whereas they decompose by a solid-state reaction at lower temperatures. The solid-state decomposition reaction is reversible for Bi2Sr2Ca1Cu2O8+delta, but not for Bi2Sr2Ca2Cu3O10+delta.