Liquid-liquid extraction of rare-earth (RE) cations has been investigated using N-dodecyldiglycolamic acid (C(12)DGAA) with a secondary amide group, and compared with that using N,N-dioctyldiglycolamic acid (DODGAA) with a tertiary amide group. C(12)DGAA enables quantitative transfer of all RE cations from moderately acidic solution, while being selective toward the heavier RE cations, and performs better than typical carboxylic-acid-type extractants. However, C(12)DGAA provides low extraction performance and separation ability for RE cations compared with DODGAA because of the weaker basicity of the amide oxygen. Slope analysis demonstrated that RE3+ transfer with C(12)DGAA proceeded through a proton-exchange reaction, forming a 1:3 complex, RE(C(12)DGAA)(3). Structural characterization by X-ray diffraction revealed that three N-butyldiglycolamic acid (C(4)DGAA) molecules coordinated to the La3+ central ion in a tridentate fashion and the La3+ primary coordination sphere consisted of three oxygen atoms from the amide group, three oxygen atoms from the ether group, and three oxygen atoms from the carboxy group.