[en] The effect of gamma-irradiation on poly uridylic acid (poly U) under various reaction conditions has been investigated. The effect of PH on the measurements of average molecular weight of poly U was first studied to choose the optimum conditions for predicting the rate constant values of strand dissociation. The G-value for single strand break (G(ssb)) was calculated and found to be 2.3. The large G(ssb)-values obtained were explained not only as resulting from the reaction of OH radicals and H-atoms on the sugar moiety, but also on their reaction with the uracil moiety which leads to chain break formation in poly U via radical transfer to the sugar moiety. Two possible mechanisms for the strand break formation could be presented. Also, the G-values were determined in the presence of oxygen. Molecular weight determinations of poly U in the presence of salts such as KCl, NaCl and NaCl O₄ using the low-angle laser light scattering technique were carried out