The interdiffusion process of n-alkane molecules in the crystal and the temperature dependence of the diffusivity are investigated in situ by optical microscopy. The optical observation of the molecular interdiffusion is possible owing to the mixing-induced solid state transition to the rotator phase; the molecularly mixed region and the low-temperature phase matrix form a sharp interphase boundary which is visible under the polarized microscope. We make a diffusion couple by a mechanical junction of single-crystalline C23H48 and polycrystalline C21H44. The chain diffusivity at each temperature is determined from the advance of the interphase boundary. The diffusivity, measured at various temperatures from 31 to 39 degrees C, shows a marked temperature dependence; the interdiffusion is rather active above 33 degrees C but it becomes remarkably inactive below 33 degrees C. An apparent activation energy for the interdiffusion above 33 degrees C is estimated to be about 300 kJ mol(-1). The mechanism of the interphase-boundary motion is discussed on the basis of the formulation devised for the moving boundary problem (the Stefan problem).