CsEuBr(3), caesium europium tribromide, crystallizes in an orthorhombic perovskite-type structure with an a(-)a(-)c(+) octahedral tilting scheme (GdFeO(3) type). CsEuBr(3) is unstable in air and one of the degradation products was identified as Cs(2)EuBr(5).10H(2)O by single-crystal X-ray analysis and synchrotron powder diffraction. The Eu(3+) ions on twofold rotational axes are coordinated by nine water molecules, and each water O atom is linked to two Br atoms by hydrogen bonds. The tricapped trigonal [EuO(9)] prisms are separated from each other by infinite {Cs(2)Br(5).H(2)O} chains; the description Eu(OH(2))(9)Cs(2)Br(5)(OH(2)) might therefore be more appropriate. The oxidation of Eu(2+) to Eu(3+) during the degradation of CsEuBr(3) is further confirmed by changes in the magnetic properties from the as-prepared material into the degraded product.