Extraction of Ce(III), Am(III), and Eu(III) from HNO₃ solutions with an acidic zirconium salt of dibutyl hydrogen phosphate (HDBP ZS) at the Zr: HDBP ratio of 1: 8 and with a mixture of HDBP ZS (1: 8) and chlorinated cobalt dicarbollide (CCD) in an o-chloronitrobenzene-CCl₄ mixture (1: 1) was studied. At a definite molar ratio of CCD and HDBP in a mixture, a considerable synergistic effect is observed, reaching a maximum (30–40 for all the examined elements) at HDBP: CCD = 2: 1. The synergistic effect observed in the system HDBP ZS (1: 8)-CCD in extraction of rare-earth and transplutonium elements can be attributed to interaction of Zr, CCD, and HDBP, leading to the formation of a complex acid that is stronger and more hydrophobic than the starting compounds. The stoichiometry of the extraction of REE and TPE with such an acid corresponds to the stoichiometry of their extraction with HDBP ZS or CCD, and the extraction ability of the system is considerably enhanced relative to the components taken separately. The metal extractabilility series corresponds to the extractability series with HDBP ZS and is opposite to the extractability series with CCD. The Zr atom in the complex extractant binds in the first coordination sphere two CCD anions, two DBP anions, and four HDBP molecules.