The distributoin of oxidation states in perovskites of the type LaA₁₋ₓ BₓO₃ (A and B transition metal ions) can be “tailored” byx variation. In paticular, in LaNiO₃ it has been shown that Fe substitution for Ni foces some Ni³⁺ into Ni²⁺, while some Fe³⁺ changes into the unusual Fe⁴⁺ state. In addition, the existence of mixed oxidation states of Fe and/or Ni in LaNi₁₋ₓ FeₓO₃ has been related to its catalytic activity in hydrogen peroxide decomposition. The Fe⁴⁺ population, obtained using Mossbauer spectroscopy, was found to be constant for all the analyzed annealing temperatures forx=0.25 concentration, where the isomer shift difference for both states is the highest and the catalytic activity is maximum.Facultad de Ciencias Exactas