Comproportionation or symproportionation is a chemical reaction where two reactants, each containing the same element but with a different oxidation number, will form a product in which the elements involved reach the same oxidation number. For example, an element A in the oxidation states 0 and +2 can comproportionate to the state +1. It is opposite to disproportionation, where two or more atoms of the same element originally having the same oxidation state react with other chemical(s) or themselves to give different oxidation numbers.
The tendency of two species to disproportionate can be determined by examining the Frost diagram of the oxidation states; if a species' value of ΔG/F is lower than the line joining the two oxidation numbers on either side of it, then it is more stable than a solution of these two species and they will undergo comproportionation.
- In lead batteries, the spontaneous reaction is:
- Pb(s) + PbO2(s) + 2 H2SO4(aq) → 2 PbSO4(s) + 2 H2O(l)
- Potassium permanganate contains manganese with oxidation number of +7 and reacts with a manganese compound having oxidation number +2 to yield manganese(IV) oxide (manganese dioxide) with oxidation number +4, potassium hydroxide and water.
- In chalcogen chemistry:
- 15 Se + SeCl4 + 4 AlCl3 → 2 Se8[AlCl4]2 
- 2 H2S(g) + SO2(g) → 3 S(s) + 2 H2O(g)
- In halogen chemistry:
- IO3- + 5 I- + 6 H + → 3 I2 + 3 H2O
- Translated from German Wiki original