Abstract: N,N-dialkyl-diglycolamic acid ligands (HDRDGA) have received considerable attention in the field of actinides/lanthanides separation research. The complexation of Ho(Ⅲ) with two analogues of HDRDGA, the water-soluble N,N-dimethyl-diglycolamic acid (HDMDGA, HA) and hydrophobic N, N-di-2-ethylhexyl-diglycolamic acid (HDEHDGA, HA'), was investigated in aqueous and organic solvent solutions at different acidity levels using potentiometry, spectroscopy, and single-crystal X-ray diffraction. Unlike typical monoprotic carboxylic acids, which coordinate to Ln(Ⅲ) solely as carboxylate anions, these two ligands can bind Ho(Ⅲ) in both deprotonated anionic and protonated neutral forms. A variety of complex species with metal-to-ligand (ligand = A⁻ and/or HA) ratios of 1∶1, 1∶2, and 1∶3 may form successively in solutions of varying acidity as the ligand concentration increases. The stability constants and molar absorptivities in UV-Vis region of these complexes was determined. Additionally, single crystals of four 1∶3 complexes, namely HoA₃, HoA₂(HA)⁺, HoA(HA)₂²⁺, and Ho(HA)₃³⁺, were successfully obtained from aqueous solutions, and their structures were analyzed by single crystal diffractometer. The changes in the stability constants of these HA complexes are qualitatively correlated and discussed in relation to the crystal structure information and compared with those of HoB₃, a complex of Ho(Ⅲ) with neutral tetramethyldiglycolamide (denoted as B). In a solvent extraction system involving HA' in 1,3-diisopropylbenzene as the organic phase and Ho(Ⅲ) in HClO₄ or HNO₃ (concentration more than 0.5 mol/L) as the aqueous phase, Ho(Ⅲ) can only be extracted into the organic phase in the form of Ho(HA')₃³⁺, with three perchlorates or nitrate ions as counter-anions. In contrast, under near-neutral conditions, complexes with varying metal-to-ligand ratios (1∶1, 1∶2, 1∶3, ligand = A'⁻) might be extracted into the organic phase, with water molecules and/or nitrate ions in the first coordination sphere of Ho(Ⅲ). For the extracted 1∶1 and 1∶2 complexes, up to two nitrate anions may directly bind to the metal center. In all complexes, whether deprotonated or not, the ligands coordinate to Ho(Ⅲ) in a tridentate fashion, similar to neutral tetraalkyldiglycolamide rather than bidentate carboxylate. The results contribute valuable insights for the applied research of such ligands.