The subsolidus phase diagram of the system Bi2O3-ZnO-Ta2O5 in the region of the cubic pyrochlore phase has been determined at 1050 degrees C. This phase forms a solid solution area that includes the ideal composition P, Bi3Zn2Ta3O14; possible solid solution mechanisms are proposed, supported by density measurements of Zn-deficient solid solutions. The general formula of the solid solutions is Bi3+yZn2-xTa3-yO14-x-y, based on the creation of Zn2+, O2- vacancies in Zn-deficient compositions and a variable Bi/Ta ratio.