NiFe1.98RE0.02O4 (where RE = La, Sm, Gd and Dy) spinel ferrites were synthesized by sol-gel autocombustion method using maleic acid as fuel. The effect of rare-earth cations (RE) doping on structural, magnetic and catalytic properties of nickel ferrite is reported. XRD and FT-IR analysis revealed the successful insertion of the rare-earth cations into the spinel matrix in all samples. We observed a decreasing of crystallite and particle sizes (estimated from XRD patterns and TEM micrographs, respectively) as well as the magnetization value in the presence of lanthanide ions into the spinel structure. Additionally, these ferrites were employed as heterogeneous catalysts for catalytic wet hydrogen peroxide oxidation (CWPO) of Orange II azo-dye. The color removal efficiency was boosted from 30.6% for undoped nickel ferrite to 90.6% for the lanthanum doped nickel ferrite, after only 15 min of reaction. The decolorization of Orange II solution followed the pseudo-first-order kinetics and was achieved via decay reaction as demonstrated by Langmuir-Hinshelwood model. The remarkable increase in catalytic performance of doped nickel ferrites with a very small amount of rare-earth cations was correlated with the size factor, expressed as the ratio between particle and crystallite size. The desirability function approach enabled to identify the optimal material (NiFe1.98La0.02O4) with the best catalytic performance; the smallest size factor and appropriate magnetic properties. (C) 2016 Elsevier B.V. All rights reserved.