Polyamide-6 (PA-6) and polyamide-4 (PA-4) doped with alkali halides were investigated by X-ray diffraction, thermal analysis, infrared and far infrared spectroscopy at various temperatures (10 K-300 K). The solubility of the alkali halides in PA-4 is better than in PA-6. Small ions are molecularly dispersed in PA-6 and PA-4. Alkali ions are coordinated to the oxygen, halide ions to the nitrogen of the amide group. The degree of crystallinity of the polyamides depends on the concentration of the alkali halides and the ion radii. Alkali halides stabilize modifications of the polyamides with not fully extended chains. In polyamides crystallized from the melt some alkali halides stabilize the y-modification or similar structures. The modification is nearly independent of the alkali halide concentration and will not be destroyed by washing out the alkali halides. If the polyamides are crystallized from formic acid, alkali halides promote a pleated sheet structure with antiparallel chains. The factor group of the layer is D, , indicated by a low temperature splitting of the % vibration modes.