Abstract: As a tridentate monocarboxylic acid extractant, N, N-bis(2-ethylhexyl) diglycolamic acid (HL) has a considerable extraction capability for trivalent lanthanides from neutral to very acidic solutions, while the extraction mechanism is different under conditions of varying acid concentrations. In order to determine the extraction behavior of HL with metal ions, the extraction of Pr(Ⅲ) with HL from nitrate media at pH=1-4 region and the coordination chemistry of Pr(Ⅲ) with N, N-dimethyl diglycolamic acid (HL′) in aqueous solutions were studied in this paper. Based on the information abstracted from the spectra and composition of the extracted complexes of Pr(Ⅲ) with HL formed at pH=1-4 region under different conditions, it is found that there are two extracted complexes for HL and Pr(Ⅲ). By comparing the spectra of the extracted complexes and the spectra of the complexes of the water-soluble homologues HL′, one of the two extracted complexes is the previously reported complex which is with a PrL₃ core, and the other one is PrL₂NO₃ which is identified for the first time. Potential titration and spectrometric titration in aqueous solution were used to study the stability constants and obtain the UV-Vis standard absorption spectra of the complexes formed between HL′ and Pr(Ⅲ). In 1 mol/L NaClO₄ solution, deprotonated HL′ can forms PrL′²⁺, PrL′⁺₂, PrL′₃ complexes with Pr(Ⅲ). With more ligands binding in the complexes, the characteristic absorption peaks of Pr(Ⅲ) in 400-600 nm region continue red-shift, and the absorption intensities just reduce slightly. Single crystals of the PrL′₃ compound are obtained by slow volatilization from aqueous solution containing Pr(Ⅲ) and L′-. The crystal structure analysis results show that in the complex the three ligands all are tridentate coordinate to Pr(Ⅲ). There are some clear difference between the absorption spectrum of PrL′⁺₂ complex in aqueous solution and the absorption spectrum of the extracted PrL₂NO₃ complex the in organic phase, the peak at 590 nm is obviously different, indicating that the nitrate in the extracted complex may be directly coordinated with Pr(Ⅲ). When the concentration ratio of the extractant to the metal ion is different, the types of extracts are different. When the ratio of the extractant concentration to the metal ion concentration is less than or equal to 2, it is inferred that nitrate participates in the coordination in the inner layer.