It is a dark brown solid that decomposes above 400 °C.

[2] Ruthenium(III) bromide can be prepared by the reaction of ruthenium metal with bromine at high temperature and pressure (720 K and 20 bar):[3] The crystal structures of ruthenium(III) bromide contain parallel (RuBr3)∞ columns.

The compound undergoes a phase transition around 384 K (111 °C) from an ordered orthorhombic structure in space group Pnmm with alternating long and short Ru-Ru distances to a disordered hexagonal TiI3-like structure in space group P63/mcm with (on average) equal Ru-Ru distances.

[1] In the disordered polymorph, the Ru-Ru distances are not believed to actually be equal but appear so due to a random distribution of two distinct column conformations.

Both polymorphs consist of hexagonally close-packed bromide ions.