This study concerns the morphology and spherulite growth rates of poly(ethylene oxide) (PEO), a semicrystalline polymer, in binary blends with poly(vinyl chloride) (PVC), an amorphous polymer, and in ternary mixtures containing (i) the amorphous polymer and (ii) tricresyl phosphate (TCP), a plasticizer for PVC. It is shown that the determining factor in the slight melting temperature depression observed in PEO/PVC blends by differential scanning calorimetry is the influence of PVC on the crystalline structure of PEO. PVC does not influence the spherulite radial growth rates of PEO, measured using polarized light videomicroscopy, in the accessible temperature range; however, TCP causes a substantial depression in the growth rates. Both additives cause the spherulite structure to become coarser and less birefringent. Unusual radial extinction bands indicate that TCP is incorporated within amorphous fold surfaces of PEO lamellae. Studies using polarized infrared microspectroscopy demonstrate that PVC causes a disordering of the orientation of the crystalline stems of PEO, and induces a higher incidence of the trans planar zig-zag conformation of the PEO chains within spherulites grown in PEO/PVC blends. Epitaxial interaction between PEO and PVC also leads to an increased incidence of the planar rig-zag conformation of the PVC. In contrast, TCP has a minimal effect on the orientation and conformation of PEO. In ternary mixtures containing PEO, PVC and TCP, the plasticization of PVC by TCP drastically reduces the effects of PVC on the microstructure of PEO.