In order to clarify the origin of the cocrystallization and phase segregation phenomena observed for a series of polyethylene blends between the deuterated and hydrogeneous species, time-resolved Fourier-transform infrared spectroscopic measurements have been performed under isothermal crystallization conditions by controlling the degree of undercooling or the temperature jump depth from the molten state to the isothermal crystallization temperature. A closeness of the crystallization rate between the pure D and H species was found to correlate well with the occurrence of the cocrystallization phenomenon. In the cocrystallizable blend sample, the crystallization rates of the D and H components were found to be almost the same and accelerated remarkably when compared with those of the individual pure components. The mechanism of cocrystallization and phase segregation was discussed from the viewpoints of both thermodynamics and kinetics.