Alkaline high-level waste (HLW) generated as a result of years of nuclear weapons production has complicated composition and requires comprehensive treatment methods, which would allow concentrating its most radiotoxic components in a small volume for geological disposal. We have investigated six alkyl-substituted o-phenylenediamine-derived sulfonamides for extraction and consecutive stripping of Sm(III) from alkaline aqueous media. Up to 81% of Sm(III) recovery at pH 13.0-13.5 was achieved by disulfonamide (dsa) or dsa/Et3N in CH2Cl2, measured after contact with organic phases and subsequent stripping with 0.1 M HNO3. The use of Et3N dramatically enhances Sm(III) extraction at lower pH ranges (10.5-11.5) but decreases extraction at pH 13.0-13.5, while control experiments with Et3N and no dsa showed no extraction. Analysis of the extraction equilibria gave a 1:1 sulfonamide-Sm(III) complexation ratio, with the extracted species also presumed to contain coordinated H2O or OH-, as also shown by DFT calculations. Titration experiments of sulfonamides with Sm(III) in CH3CN were consistent with a 1:1 complexation ratio of dsa-6 to Sm(III) with a K11 = 6.6 × 107 M-1 derived from nonlinear regression analysis of the 1:1 binding isotherm. Theoretical DFT calculations determined the structures of possible species formed during extraction and the thermodynamics of extraction processes based on several initial [Sm(OH)y(NO3)z(H2O)x]3-y-z species and 1:1 Sm(III)/dsa-32- complexes formed in the organic phase, in which dsa complexes to Sm(III) in its bis-deprotonated form (denoted below as dsa-32-). Organization of close ion pairs of type {Na[Sm(dsa-32-)(OH)2]·2H2O} was shown to be thermodynamically favorable for extraction from alkaline aqueous media with pH = 13.0-13.5. Theoretical calculations also demonstrated thermodynamically favorable coordination to Am(III).