The objective of this work was to investigate the effect of additions of petroleum coke to coal blends on the quality of the interfaces between the petroleum coke and reactive coke components in the resulting cokes and on the coke friability. Two medium volatile coals and one delayed petroleum coke were used. The variables studied were the petroleum coke particle size, the amount added and the effect of heat treatment of the petroleum coke. With 10 wt% petroleum coke in the blend the quality of the interfaces decreased as the size of the petroleum coke increased from < 212 mm to 1 mm to 2 mm. For the smallest size petroleum coke, as the concentration was increased up to 40 wt%, the coke friability progressively improved in line with increases in interface quality. Heat treating the delayed petroleum coke to 1000 degrees C, prior to blending and carbonisation, resulted in an increase in the number of poor quality interfaces in the coke.