It is established that solid diphenyl[dibutylcarbamoylmethyl]phosphine oxide (DPDBCMPO) forms liquid phases ("liquid reagent") immiscible with acid solutions on contact with mineral acids. It is found that the compositions of "liquid reagent" correspond to DPDBCMPO.HNO3nH2O and 2DPDBCMPO.HAn.nH2O, where HAn is HClO4 or HCl. The compositions of nitrate and perchlorate complexes of "liquid reagent" with UO22+ and Nd3+ are found. Extraction of a number of actinides in various oxidation states as well as extraction of some lanthanides by "liquid reagent" in the absence of organic solvent is studied. It is shown that in case of extraction by "liquid reagent" distribution ratios increase in comparison with usual solvent extraction. The complex of “reagent” with nitric acid has redox properties resulting in a change of valence states of some actinides in the process of extraction. The use of inert matrices containing DPDBCMPO is suggested for isolation and immobilization of actinide and rare-earth elements by vitrification.