Cationic dyeable polycaproamide (CD-PCA) and cationic dyeable poly(trimethylene terephthalate) (CD-PTT) polymers were extruded (in ratios 75/25, 50/50, and 25/75) from one melt twin-screw extruders to prepare three CD-PCA/CD-PTT polyblended polymers and then spin fibers. This work examines the rheology, thermal characteristics, crystallinity, tenacity, density, and miscibility parameter p value of CD-PCA/CD-PTT polyblended polymers and fibers using gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, potentiometer, rheometer, the density gradient analysis, wide-angle X-ray diffraction, and extension stress-strain measurement. The melting behavior of CD-PCA/CD-PTT polyblended polymers revealed negative-deviation blends (NDB). The 50/50 blend of CD-PCA/CD-PTT had the lowest melt viscosity. The experimental DSC results demonstrated that CD-PCA and CD-PTT molecules constituted an immiscible system. In particular, CD-PCA, CD-PTT, and their polyblended fibers yielded a double endothermic peak. The tenacity of CD-PCA/CD-PTT polyblended fibers initially declined and then increased as the CD-PTT content increased. The crystallinities and densities of CD-PCA/CD-PTT polyblended fibers were linearly related to the blend ratio. The values of the miscibility parameter p for all CDPCA/CD-PTT samples were under zero, revealing electrostatic repulsion between CD-PCA and CD-PTT molecules. All experimental data supported the immiscibility of CDPCA/CD-PTT polyblended fibers. (c) 2007 Wiley Periodicals, Inc.