The phosphonium (more obscurely: phosphinium) cation describes polyatomic cations with the chemical formula . Salts of the parent PH₄⁺ are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakis(hydroxymethyl)phosphonium chloride:

PH₃ + HCl + 4 CH₂O → P(CH₂OH)₄⁺Cl⁻