The hydrogen isotope radiolytic yields, G(H2), G(HD) and G(D2) were determined in H2O/D2O mixtures under chemical conditions close to a LOCA in a PHWR like Atucha I Nuclear Station, that is 2·10–3 MH3BO3 and p(H+D)=8.5±0.2. The total hydrogen radiolytic yield G(H2+HD+D2) as a function of the deuterium atom fraction goes through a flat maximum at about 0.58. This result in dicates that the 4% flammability limit for hydrogen in the reactor's containment with be reached sooner than what is expected assuming a linear combination of pure H2 and D2 radiolytic yields. Hydrogen radiolytic production in 10–3 M KBr in H2O/D2O mixtures gives the same results as in the boric solutions suggesting a bimolecular B(OH)
+OH reaction. Identical isotope concentration factors were calculated for both solutions.