Disproportionation or dismutation disproportionation is any chemical reaction of the type A + A → A' + A", where A, A' and A" are different chemical species. For example: 2 ArH+ → ArH + ArH2+ The reverse of disproportionation is called comproportionation. This definition comes from IUPAC (http://www.iupac.org/goldbook/D01799.pdf) While the most common type is a redox reaction, other types are possible.