Methylphosphine is the simplest organophosphorus compound with the formula CH₃PH₂, often written MePH₂. It is a malodorous gas that condenses to a colorless liquid. It can be produced by methylation of phosphanide salts:^[1]

KPH₂ + MeI → MePH₂ + KI

The compound exhibits the properties characteristic of a primary phosphine, i.e., a compound of the type RPH₂. It can be oxidized to methylphosphonous acid:

MePH₂ + O₂ → MeP(H)O₂H

It protonates to give a phosphonium ion:

MePH₂ + H⁺ → MePH₃⁺

With strong bases, it can be deprotonated to give methylphosphanide derivatives:

MePH₂ + KOH → K[MePH] + H₂O