Half life of56Mn in Mn, MnCl2.4H2O, KMnO4, MnO2, MnSO4.4H2O and that of128I in I2, NaI, KI, KIO4, HIO4, KIO3 was measured. The nuclides were produced by thermal neutron bomabardment in241Am-Be neutron source and their gamma activities were measured using a NaI/T1/ detector. Half life of56Mn varied with the change in oxidation state. No such systematic variation was observed for the half life of128I but it is affected by the nature of cation or anion attached to it. The change in decay constant, /
was also calculated.