A bismuth-213 generator comprising an insoluble composition having the general formula Zr(Phosponate).sub.x(HPO.sub.4).sub.2-x.nH.sub.2O, wherein x is between 0 and 2; and n is the number of waters of hydration; and wherein cations of radioactive isotopes selected from radium, actinium and combinations thereof are immobilized on the composition. The value of x may be between about 0.2 and about 1. The phosphonate may be n-phosphonomethyl-miniodiacetic acid (PMIDA), wherein x may be between about 0.1 and about 1.9. The phosphonate may be one or more phosphonate having the formula: H.sub.2O.sub.3P--(CH.sub.2).sub.a--N--((CH.sub.2).sub.bCO.sub.2H)--((CH.s- ub.2).sub.cCO.sub.2H), wherein a, b, and c are numbers from 1 to 3 that may or may not be equal. The value of x may also be between about 0.1 and 1.9.