The thermal stability of the (Bi,Pb) 2 Sr 2 CaCu 2 O 8 + δ phase in air has been studied by means of X-ray diffraction, SEM observations, DTA/TG and ac susceptibility measurements. It was found that the Pb segregation previously reported to occur around 700°C is reversible upon further annealing in air at a higher temperature. It is shown that the kinetics of the segregation reaction are slow. The small structural modification in the (Bi,Pb) 2 Sr 2 CaCu 2 O 8 + δ phase induced by the change of Pb concentration is evidenced.