Many of methylenes electronic states lie relatively close to each other, giving rise to varying degrees of radical chemistry. The ground state is a triplet radical with two unpaired electrons (X̃B), and the first excited state is a singlet non-radical (ãA'). With the singlet non-radical only 38 kJ above the ground state, a sample of methylene exists as a mixture of electronic states even at room temperature, giving rise to complex reactions. For example, reactions of the triplet radical with non-radical species generally involves abstraction, whereas reactions of the singlet non-radical not only involves abstraction, but also insertion or addition.
:(X̃B) +  →  + [HO]
:(ãA) +  →  +  or 
The singlet state is also more stereospecific than the triplet.
Unsolvated methylene will spontaneously autopolymerise to form various excited oligomers, the simplest of which, is the excited form of the alkene ethylene. The excited oligomers, decompose rather than decay to a ground state. For example, the excited form of ethylene decomposes to acetylene and atomic hydrogen.
:2  →  → HCCH + 2 H
Unsolvated, excited methylene will form stable ground state oligomers.