Disproportionation is a reaction in which two units (atoms, molecules, ions, radicals) of the same species react with each other to give two units (atoms, molecules, ions, radicals) of different species. Disproportionation has the following general equation.
Dismutation is a disproportionation in which one reacting unit undergoes oxidation and the other reduction. Thus, the above example of disproportionation is also a dismutation, whereas autoionization of water, shown below, is a disproportionation but not a dismutation.
Dismutation, therefore, is a subset of disproportionation: All dismutations are disproportionations, but not all disproportionations are dismutations.