Crystallinities of films of copolymers of chlorotrifluoroethylene with 0 to 4% vinylidene fluoride, different molecular weights and thermal histories were calculated from differential scanning calorimetry (DSC), X-ray diffraction (XRD), and density measurements. The crystallinities for these films range between 16 to 58%, 11 to 71%, and 13 to 77% as determined by DSC, XRD, and density, respectively. Although the numerical value of the DSC, XRD, and density crystallinities obtained by each of the techniques are different, they correlate well with each other. The primary source of error in DSC is the uncertainty in drawing the baseline for separating the heat of fusion and heat of crystallization. In XRD, the ambiguities in the separation of the amorphous and crystalline peaks reduce the precision of the results. Calculation of crystallinity from density values requires that the distribution of the VF2 between the amorphous and crystalline regions be known. In addition, the density technique is not sensitive to variations in the crystalline and amorphous densities.