A series of oriented polypropylene tapes each containing carbon black having defined properties and at a specified concentration were produced and tested for their tensile and physical behaviours at 20 and 130 degrees C. Tenacities at 20 degrees C decreased as carbon black concentration increased from 2.5 to 5% (w/w) and with increasing particle size. At constant particle size and concentrations, tape tenacities were also inversely dependant on carbon black structure. The observed effects were expressed in terms of occlusion of polymer within carbon black aggregates having high structures thereby increasing the pigment particles effective volumes. When tested at 130 degrees C, variation in tape tenacities were less, probably as a consequence of the reordering effects of annealing. DSC and IR measures of crystallinity showed that the presence of carbon black and variations in particle properties influenced the character of the crystalline regions.