Polycaproamide (PCA) polymer and cationic dyeable polycaproamide (CD-PCA) polymer were blended mechanically in the proportions of 75/25, 50/50, and 25/75 in a melt twin-screw extruder to prepare three PCA/CD-PCA polyblended polymers. Blends of PCA and CD-PCA were spun into filaments. The molar ratio of 5-sodium sulfonate dimethyl isophthalate (5-SSDMI) for CD-PCA polymer was 1.99%. This study investigated the physical properties of PCA/CD-PCA polyblended filaments using gel permeation chromatograph (GPC), nuclear magnetic resonance (NMR), gas chromatography (GC), potentiometer, thermogravimetric analysis (TGA), a rheometer, differential scanning calorimetry (DSC), the density gradient method, wide-angle X-ray diffraction (WAXD), extension stress-strain measurement, and scanning electron microscope (SEM). Flow behavior of PCA/CD-PCA polyblends exhibited positive-deviation blends (PDB), and the 50/50 blend of PCA/CD-PCA showed a maximum value of the melt viscosity. Experimental results of the DSC indicated PCA and CD-PCA molecules easily formed miscible domains. The crystallinities of PCA/CD-PCA polyblended filaments decreased as the 5-SSDMI content increased. Tenacities/densities of PCA/CD-PCA polyblended filaments were also found to decline as the 5-SSDMI content increased. The surface of PCA/CD-PCA polyblended filament exhibited a uniform morphology from the SEM data. PCA and CD-PCA polymers were proved to be a compatible system. The weight loss percentages of the PCA/CD-PCA polyblended filaments increased as the 5-SSDMI content increased in aqueous NaOH solution. The porous morphology of a larger size from 0.01 to 2 mu m in diameter was observed after alkali treatment of PCA/CD-PCA polyblended filament. (c) 2005 Wiley Periodicals, Inc.