Herein is presented a rare example of salt/ cocrystal polymorphism involving the adduct between ethionamide (ETH) and salicylic acid (SAL). Both the salt and cocrystal forms have the same stoichiometry and composition and are both stable at room temperature. The synthetic procedure was successfully optimized in order to selectively obtain both polymorphs. The two adducts' structures were thoroughly investigated by means of singlecrystal X-ray diffraction, solid-state NMR spectroscopy, and density functional theory (DFT) calculations. From the solidstate NMR point of view, the combination of mono-and multinuclear experiments ( 1 H MAS, 13 C and 15 N CPMAS, 1 H-{ 14 N} D-HMQC, 1 H− 14 N PM-S-RESPDOR) provided undoubted spectroscopic evidence about the different positions of the hydrogen atom along the main N•••H•••O interaction. In particular, the 1 H− 14 N PM-S-RESPDOR allowed N−H distance measurements through the 1 H detected signal at a very high spinning speed (70 kHz), which remarkably agree with those derived by DFT optimized X-ray diffraction, even on a natural abundance real system. The thermodynamic relationship between the salt and the cocrystal was inquired from the experimental and computational points of view, enabling the characterization of the two polymorphs as enantiotropically related. The performances of the two forms in terms of dissolution rate are comparable to each other but significantly higher with respect to the pure ETH.