Gradient structures, produced by interdiffusion in microlayers of a high density polyethylene/linear low density polyethylene polymer pair that cocrystallizes isomorphically, were studied experimentally. Microlayers were taken into the melt for a period of time, and the compositional gradient was fixed by crystallization upon quenching. High specific interfacial area of microlayers offset the low diffusion mobility of polymeric chains so that the microlayer in the melt approached compositional homogeneity on a laboratory time scale. Taking advantage of the systematic change of the melting temperature with the blend composition, the compositional gradient was visualized by progressively melting the microlayer with increasing temperature. This made it possible to monitor the kinetics of interdiffusion without using a chemical label. The compositional profiles were analyzed with a diffusion model formulated for a polydisperse system. Diffusion coefficients for lightly branched and linear polyethylene chains, which correlated well with the data of previous studies, were obtained. It was found that the interlayer boundaries remained stationary during a characteristic time of interdiffusion of the component main fractions, and moved at long times as high molecular weight fractions became involved in interdiffusion. The moving boundary phenomenon was investigated with optical and atomic force microscopy and the development of crystalline morphology in the microscopic compositional gradient was described.