This paper describes the polymorphic transitions of Bi(0.775)Ln(0.225)O(1.5) (Ln = La-Er and Y), which is a representative composition of a rhombohedral solid solution over a limited range inside the 20-30 mol% Ln(2)O(3). Three kinds of polymorphs are found : a hexagonal low-temperature modification (beta(2)) isomorphous with Bi0.765Sr0.235O1.383 (BiSrO) type, a hexagonal intermediate (beta(1)), and the well known high-temperature modification (delta) isomorphous with the fluorite type. The way the beta(2) phase behaves on heating can be divided into two groups depending on the Ln(3+) ion : (1) one or two reversible transitions from beta(2) to beta(1) or from beta(2) to beta(1) then from beta 1 to delta for La, Pr, Nd, Sm, Eu and Gd; and (2) an irreversible transition from beta(2) to delta for the others. The thermal behaviour in group (1) closely resembles the BiSrO-type solid solution in the systems Bi2O3-MO (M = Ca, Sr and Pa).