The interdiffusion of n-alkane molecules in the crystal state is investigated in the binary system of C21H44 and C23H48. A mechanical junction between a single crystal of C-23 and polycrystalline C-21 is held at a temperature just above the transition to the rotator phase in C-21, and optical changes along the junction surface are studied in situ. Direct observation of the interdiffusion process is enabled by virtue of the fact that the interfacial region at the junction makes a mixing-induced transition to the rotator phase, which shows marked differences in optical properties from the surrounding low-temperature phase. Two ways of forming the mechanical junction are tested : the crystal of C-21 is placed either on the side surface of the C-23 single crystal (lateral junction) or on the upper surface of the C-23 Single crystal (upper junction). Both transverse chain diffusion within the lamella and longitudinal diffusion along the chain axis are studied separately. The mixed region and the low-temperature phase matrix are found to have a rather sharp boundary (diffusion front), and the interdiffusion rate is estimated quantitatively from the advance of the diffusion front. The transverse diffusion front at the lateral junction is found to advance by a distance that is proportional to the square root of time, which is clearly diffusion-controlled behaviour. The estimated diffusion constant shows fairly good correspondence with the self-diffusion coefficient determined by a tracer method. In the case of the upper junction, the molecules show longitudinal diffusion into deep layers followed by transverse diffusion therefrom. The transverse diffusion front is nearly circular; diffusion is considered to be nearly isotropic within the a-b plane. The transverse diffusion in this case is severely restricted by the matrix.