Biobased polyamides synthesized from diaminoisoidide (DAII), 1,4-diaminobutane, and sebacic acid are investigated by FT-IR., C-13{H-1} magic-angle spinning/cross-polarization (CP/MAS) NMR spectroscopy, and WAXD. Their molecular conformation and mobility undergo distinct changes as a function of temperature and diaminoisoidide content in the compositions. The presence of randomly distributed diaminoisoidide (DAII) in the polyamides reduces their hydrogen bond density and affects the trans/gauche conformer population. FT-IR and C-13{H-1} CP/MAS NMR were successfully employed to prove the incorporation of DAII both in the crystalline and amorphous phase of these materials. Changes in the position and intensities of the diaminoisoidide-assigned FT-IR and NMR signals prove that the DAII monomer causes a conformational disorder of the polyarnides and thus influences the crystal structure of these materials, as also shown by WAXD experiments. Extensive ab initio calculations using a combination of DFT and MP2 methods reveal a number of stable conformers of DAII and thus an unexpectedly high flexibility of the isohexide unit.