Poly(vinylidene fluoride) and vinylidene fluoride-tetrafluoroethylene copolymers cocrystallize depending on the structures present in each. In the alpha-phase, the maximum difference in the defect concentration of the two polymers (DELTA defect) equals 12.3% during isothermal crystallization, and is equal to 10.5% when the system is quenched. In the beta-phase, it is 11.8% for isothermal crystallization, and approximately 5.5% when quenched. Apart from isomorphic cocrystallization between components of the same polymorph, cocrystal formation between the alpha- and beta-polymorphs was also observed. The resulting cocrystal adopts the structure of a particular polymorph, according to the results obtained from potential energy calculations. The interplanar spacings (d(hkl)) of the alpha-phase of PVF2 do not change with the H-H defect concentration, but for the beta-phase d200 increases with the defect concentration. The reason for this difference in behaviour has been attributed to the more compact nature of the beta-polymorph unit cell, when compared to that of the alpha-polymorph. The limiting value of the DELTA defect for cocrystallization in the alpha-phase has been attributed to the larger difference in the intramolecular potential energy, whereas that of the beta-phase has been attributed to the compact nature of the unit cell.