Spinel-type oxides of MgMnxFe2-xO4, x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, were synthesized as a solid state reaction and characterized with dilatometry and resistivity measurements up to 1000 degrees C. Results showed a general decrease of the linear expansion and an increase in conductivity as a function of the Mn content. Point electrodes were analyzed from 300 to 600 degrees C in a pseudo-three-electrode setup in 1% NO, 1% NO2, and 10% O-2 using cyclic voltammetry. The activities in O-2 were in general very low whereas the activities in NO were slightly higher. The activities in NO2 were for all materials much higher than the activities in O-2. Even though Mn tends to decrease the activity of the materials, current ratios of I-NOx/IO2 have relatively high values in both NO and NO2 .