The objective of this work was to investigate methods for increasing the impact resistance of composite sheets made from glassfiber mat-reinforced polypropylene prepeg by varying its thermal history. A 60:40 (wt %) mixture of woven glass fiber-PP was crystallized at various temperatures and times to examine the effect of the thermal history (during cooling fi om the melt) on the degree of crystallinity and spherulitic morphology and to study the relationship between these factors and mechanical properties. The composite laminates were manufactured within a flat mold using a compression molding press and then crystallized from the melt in the range 106-156 degreesC for 10, 30, 60, and 240 min in an air oven. The degree of crystallinity that developed in the matrix polymer was determined using differential scanning calorimetry (DSC), and the matrix morphology was examined by permanganic etching followed by scanning electron microscopy. The highest peak and failure energies during impact were achieved when maximum crystallinity was produced in the specimen crystallized at 134C for 4 h. Electron microscopy of etched interior sections of impacted specimens has enabled a more detailed understanding of the impact behavior of these materials. The greatest improvement appears to result from an. increase in the propensity of cracks to propagate along spherulitic boundaries by virtue of mechanisms facilitated by the results of the differential contraction of the crystalline and amorphous phases within the polypropylene. Differential contraction of the glass and polypropylene appears to be a less important factor, although voids created by such processes have to be taken into account.