Cationic dyeable nylon 6 (CD-N6) and cationic dyeable poly(ethylene terephthalate) (CD-PET) polymers were extruded (in the proportions of 75/25, 50/50, 25/75) from one melt twin-screw extruders to prepare three CD-N6/CD-PET polyblended polymers and then spin filaments. The molar ratio of 5-sodium sulfonate dimethyl isophthalate (5-SSDMI) for CD-N6 and CD-PET polymers were 2.01% and 2.04%, respectively. This study investigated the flow and thermal characteristics of CD-N6/CD-PET polyblended polymers and filaments using gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), potentiometer, a rheometer, the density gradient, wide-angle X-ray diffraction (WAXD), and extension stress-strain measurement. Flow behavior of CD-N6/CD-PET polyblended polymers exhibited negative-deviation blends (NDB), and the 50/50 blend of CD-N6/CD-PET showed a minimum value of the melt viscosity. Experimental results of the DSC indicated CD-N6 and CD-PET molecules formed an immiscible system. Particularly, a double endothermic peak was observed from CD-N6, CD-PET and their polyblended filaments. The tenacity of CD-N6/CD-PET polyblended filaments decreased initially and then increased as the CD-PET content increased. Crystallinities and densities of CD-N6/CD-PET polyblended filaments were the linear relation with the blend ratio. The miscibility parameter p values of CD-N6/CD-PET all samples were less than zero. It indicated the electrostatic repulsion was evident between CD-N6 and CDPET molecules. From the experimental data, the CD-N6 and CD-PET polymers were identified to be immiscible. (c) 2006 Wiley Periodicals, Inc.