In Bi5Pb3O10.5 there exist three polymorphs: the low-temperature stable beta (L) the high-temperature stable beta which shows a high oxide-ion conduction, and the metastable beta (2) phases. This paper describes the phase transformations between these polymorphs using TG-DTA and de conductivity measurements. The triclinic beta (L) phase transforms to the anti-alpha -AgI-type bce beta phase at about 585 degreesC; on cooling, at about 560 degreesC the beta phase changes to the tetragonal beta (2) phase which is kept to room temperature. The beta (2) phase reverts gradually to the beta (L) phase during heating. The rate of this gradual beta (2)-to-beta (L) transition depends strongly on a heating rate. A very small weight change is observed with respect to the beta phase: weight gain upon the beta (L)-to-beta transition in the heating direction and weight loss upon the beta -to-beta (2) transition in the cooling direction. However, the TG-DTA results under N-2 gas atmosphere suggest that this weight change seems to be due to the excessive oxygen vacancies of the beta phase.