in order to clarify the origins of cocrystallization and phase segregation phenomena observed for blend samples between fully-deuterated high-density polyethylene (DHDPE) and hydrogeneous linear low-density polyethylene (LLDPE) with various degrees of ethyl branchings, diffusion process of deuterated (D) and hydrogeneous (D) polyethylene chains through the contact interface between the D and H films was traced in situ by time-resolved Fourier-transform infrared spectroscopic measurement at temperatures above the melting points, from which the diffusion coefficients were evaluated. Spatial distribution of D and H chains in the interfacial region of the contacted films was also observed at room temperature by infrared spectral measurement for the melt-quenched samples: the homogeneous mixing was observed for a pair of DHDPE and LLDPE(2) with 17 ethyl branchings per 1000 carbon atoms and the heterogeneous distribution for a pair of DHDPE and LLDPE(3) with 41 ethyl branchings per 1000 carbon atoms. From the spatial distribution data the diffusion coefficients were evaluated again, consistent with that obtained by the time-resolved experiment. Activation energy for the diffusion motion of chains was also estimated from the temperature dependence of the diffusion coefficient. A large difference in diffusion coefficient and activation energy was found between the above-mentioned two sample systems, relating well with the difference in the aggregation state of D and H chains in the melt as well as in the crystallized sample: cocrystallization phenomenon for DHDPE/LLDPE(2) and phase segregation for DHDPE/LLDPE(3).