Decay rate /Change of decay rate /' 5 Zn Summary The relative decay rate change, Δ λ/λ, of 65 Zn in its two oxidation states, Zn° and Zn 2 is determined by measuring gamma activities of a pair of samples alternately. The redox reaction of Zn is realized in a secondary battery at a certain time interval during the activity measurement. By using the so called "jumpmçthod" the Δλ/λ value for the given set of oxidation states is determined to be Δλ λ( 45 Ζη° ) -\(Ca 65 Zn(OH) 4 ) (11.5 ± 0.4) X 10-λ λ(« 5 Ζη°)The theoretical estimates made in this work corroborate approximately the experimental values.