The purpose of this study was to characterize two polypropylene copolymer materials which have similar melt viscosities (melt indices) but behave very differently both during and after processing into fibers. Although the two materials were generated by two different synthetic routes, it was felt that the materials should process similarly because of the similar melt viscosities. The first material, a reactor product, processed normally. With the second material, a physical blend, the spinline tended to break at the spinneret, shutting down the fiber spinning process. These materials are analyzed using optical microscopy (OM), differential scanning calorimetry (DSC) and thermomechanical analysis (TMA), to examine the source of excessive spin breakage. The source of the spinline breakage is found to be "gels" or globular masses on the otherwise smooth surface of the physical blend fibers. Several observations of physical characteristics, such as melting behavior and tensile stiffness, show that the two materials differ in crystallization rates, fraction, and orientation of crystals, and internal stress states. Much of the evidence points to differences in microstructure, molecular weight and molecular weight distribution between the two materials as the root cause of the differences in processability and performance. This conclusion is supported by gel permeation chromatography recently performed on the materials.