Using an X-ray quantitative phase analysis, the effect of adding 2% polyethylene terephthalate on the course of structure changes of two orthocoking coals differing in maximum fluidity during their common carbonization was studied. Interplanar distances, height of crystallite, and quantity of crystalline phase were determined for the co-pyrolyzates obtained. The transformation of the texture of the material of the pore walls of co-pyrolyzates was investigated using scanning electron microscopy. In co-pyrolyzates, an addition of polyethylene terephthalate to coal of higher maximum fluidity was found to favor the formation of structure and texture of parameters similar to pyrolyzates of coal of lower maximum fluidity.