We investigated (Fe,Mn)-Mg interdiffusion along the  axis in single crystals of natural diopside at ambient pressure, at temperatures between 1000°C to 1200°C and at oxygen fugacities between 10−18.5 atm to 10−6 atm. Thin layers (~350–550 Å) of ferro-johannsenite deposited by RF plasma sputtering provided iron and manganese for exchange with magnesium from the diopside substrates. Interdiffusion profiles of Fe-Mg and Mn-Mg were analysed by secondary ion mass spectrometry in depth profiling mode. We found that the (Fe,Mn)-Mg interdiffusion coefficient is strongly pO2 sensitive. At 1100°C and between 10−7 atm and 10−15 atm the interdiffusion coefficient is proportional to ~(pO2)m, indicative of an extrinsic point defects regime. We obtained m = 0.22 ± 0.02, indicating a vacancy mechanism. The data at 1100°C further suggest that at pO2 × 10−15 atm and at pO2 > 10−7 atm the interdiffusion coefficient is pO2-independent, possibly in relation with the appearance of Early Partial Melting (EPM, Jaoul & Raterron, 1994). At constant oxygen fugacity and without EPM the interdiffusion coefficient has an activation energy of 297 ± 31 kJ/mol. Our data significantly improve the constraints on the activation energy beyond the preliminary results reported earlier (Dimanov & Sautter, 2000). This work contributes towards a better understanding of the point defects chemistry and diffusion mechanisms in Fe-bearing diopside.