The elastic compliances of polyethylene (PE) and poly(vinylalcohol) (PVA) crystals were measured by X-ray diffraction using the films prepared by gelation/crystallization from dilute solutions. The measurements were carried out at 20 degrees C for the PE and PVA films with elongation ratios of 60 and 11, respectively. For the PE films, the measurable compliances, S-13(c) and S-23(c), were (-0.013 to -0.017) x 10(-2) GPa(-1) and (-0.040 to 0.043) x 10(-2) GPa(-1), respectively, while for the PVA films, they were -0.012 x 10(-2) and (-0.037 to -0.038) x 10(-2) GPa(-1), respectively. From the above values, the corresponding Poisson's ratios, v(13)(c) and v(23)(c), Of PE crystals were estimated to be 0.029-0.038 and 0.090-0.096, respectively, while v(13)(c) and v(23)(c), for PVA crystals are 0.026 and 0.081-0.083, respectively. A question can be arisen as to whether the crystal elastic compliance and Poisson's ratio as measured by X-ray diffraction are equal to the corresponding intrinsic values. To check this, a mathematical representation based on a linear elastic theory was proposed by which one may investigate the dependence of the crystal Poisson's ratios on molecular orientation and crystallinity. The mathematical description indicated that the crystal Poisson's ratio as estimated by X-ray diffraction are essentially different from the intrinsic crystal Poisson's ratios. Even so, the numerical calculations indicated that the values as measured by X-ray diffraction are equal to the intrinsic values, if the morphology of test specimens can be represented as a parallel model for composite of the crystal and amorphous phases, approximately.