[en] Absorption spectrophotometric and X-ray powder diffraction methods have been applied to a study of the trichlorides of ²⁴⁹Bk and ²⁴⁸Cf and their relationship through the β decay of ²⁴⁹Bk. BkCl₃ has been prepared for the first time in the PuBr₃-type orthorhombic modification by quenching from the melt. Each of the crystal forms (UCl₃-type hexagonal and PuBr₃-type orthorhombic) of BkCl₃ and CfCl₃ has been characterized on the basis of its solid-state absorption spectrum. The orthorhombic forms of BkCl₃ and CfCl₃ are the high-temperature modifications with respect to the hexagonal phases, with the apparent transition temperatures near the melting points of BkCl₃ (876 K) and CfClnumber (818 K). Orthorhombic BkCl₃ transmutes to orthorhombic CfCl₃ and hexagonal BkCl₃ transmutes to hexagonal CfCl₃. Thus, it was found that both the oxidation state and the crystal structure of the parent ²⁴⁹Bk compound were retained by the daughter ²⁴⁹Cf compound through β decay in the bulk phase solid state. 11 references, 5 figures