Tetrakis(hydroxymethyl)phosphonium chloride (THPC) is an organophosphorus compound with the chemical formula [P(CH2OH)4]Cl.

It is a white water-soluble salt with applications as a precursor to fire-retardant materials[1] and as a microbiocide in commercial and industrial water systems.

THPC can be synthesized with high yield by treating phosphine with formaldehyde in the presence of hydrochloric acid.

[7] Tris(hydroxymethyl)phosphine can also be used to synthesize the heterocycle, N-boc-3-pyrroline by ring-closing metathesis using Grubbs' catalyst (bis(tricyclohexylphosphine)benzylidineruthenium dichloride).

(Z = CONH2) Similar reactions occur when THPC is treated with acrylic acid; only one hydroxymethyl group is displaced, however.