The formation and the thickness growth of the (Bi,Pb)(2)Sr2Ca2Cu3O10 phase in Ag-sheathed tapes have been investigated by scanning electron microscopy on samples sintered at 840 degrees C in a flow of 8.5% O-2 (rest N-2) and quenched in air after sintering for 1 to 50 h. The thickness of the (Bi,Pb)(2)Sr2Ca2Cu3O10 grains was measured statistically after different sintering times. The experimental results show that the average thickness of these grains increases during the entire sintering period, while the average thickness growth rate decreases exponentially with sintering time. The volume fractions of the various phases present during the heat treatment were also measured from micrographs. Detailed studies of the microstructure and phase formation kinetics support the view that the formation of the (Bi,Pb)(2)Sr2Ca2Cu3O10 phase proceeds via a nucleation and growth process. Based on the present observations, a model describing the microstructure evolution is presented. (c) 2005 Springer Science + Business Media, Inc.