Display omitted ► Cr substitution improved Fenton degradation of methylene blue and acid orange II. ► The extent of improvement by Cr substitution varied with the substitution level. ► The degradation of two dyes followed different mechanism and kinetics equations. ► This study is a benefit for the application of magnetite in treating wastewater. In this study, the heterogeneous Fenton degradation of cationic and anionic dyes catalyzed by a series of Cr-containing magnetites (Fe3−xCrxO4, x=0.00, 0.18, 0.33, 0.47 and 0.67) has been investigated under neutral pH conditions. Methylene blue (MB) and acid orange II (AOII) were chosen as models of cationic and anionic contaminants. Emphases were laid on the comparison of degradation characteristic between MB and AOII and the effect of Cr substitution on the degradation efficiency of both dyes. The octahedral occupancy of Cr3+ increased the BET surface area and superficial hydroxyl amount of magnetite, resulting in an improvement of adsorption ability of MB. However, these Cr-containing magnetites showed no adsorption to AOII. The MB degradation, following the Langmuir–Hinshelwood model, was well fitted by zero-order equation while AOII degradation following the Eley–Rideal model, was well fitted with two-stage pseudo-first-order kinetics. The Cr incorporation significantly improved the catalytic activity of magnetite in heterogeneous Fenton reaction, but the extent of improvement varied with the substitution level. These new insights are of high importance for the environmental application of metal substituted magnetites in the purification of textile wastewater.