Phosphorus pentabromide is a reactive, yellow solid of formula PBr5, which has the structure [PBr4]+Br− (tetrabromophosphonium bromide) in the solid state but in the vapor phase is completely dissociated to PBr3 and Br2.

Rapid cooling of this phase to 15 K leads to formation of the ionic species phosphorus heptabromide (tetrabromophosphonium tribromide [PBr4]+[Br3]−).

[2] It can be used in organic chemistry to convert carboxylic acids to acyl bromides.

It strongly irritates skin and eyes.

[1] It decomposes above 100 °C to give phosphorus tribromide and bromine:[3] Reversing this equilibrium to generate PBr5 by addition of Br2 to PBr3 is difficult in practice because the product is susceptible to further addition to yield phosphorus heptabromide [PBr4]+[Br3]−.