The oxidative dehydrogenation of isobutyronitrile was studied using an iron phosphate with a P/Fe atomic ratio of 1.2 as the catalyst. The main products were methacrylonitrile, acetone, and carbon oxides with a small amount of cyanic acid. The selectivities to methacrylonitrile were about 70 mol-% at an isobutyronitrile conversion of around 85% in the presence water vapor. On the other hand, in the absence of water vapor, the selectivity reached 80 mol-% and a marked amount of carbon deposit was observed, though the activity was relatively stable. The activity of a fresh catalyst consisting of only Fe3+ ions is low, but it increases with an increase in the time-on-stream up to about 10 h and then levels off with a further increase. It was also found that the selectivity to methacrylonitrile increases at the expense of a decrease in acetone, as the reaction temperature rises. The variations in the structures and valence states of iron ions due to the use in the reaction and their effects on the catalytic performances were also studied.