BaCl2 crystallizes in two forms (polymorphs). One form has the cubic fluorite (CaF2) structure and the other the orthorhombic cotunnite (PbCl2) structure. Both polymorphs accommodate the preference of the large Ba2+ ion for coordination numbers greater than six.[6] The coordination of Ba2+ is 8 in the fluorite structure[7] and 9 in the cotunnite structure.[8] When cotunnite-structure BaCl2 is subjected to pressures of 7–10 GPa, it transforms to a third structure, a monoclinic post-cotunnite phase. The coordination number of Ba2+ increases from 9 to 10.[9]

In aqueous solution BaCl2 behaves as a simple salt; in water it is a 1:2 electrolyte and the solution exhibits a neutral pH. Its solutions react with sulfate ion to produce a thick white precipitate of barium sulfate.

Ba2+ + SO42− → BaSO4

Oxalate effects a similar reaction:

Ba2+ + C2O42− → BaC2O4

When it is mixed with sodium hydroxide, it gives the dihydroxide, which is moderately soluble in water.