Interdiffusion and interfacial reaction between a sputter‐deposited YBa2Cu3Ox thin film and substrates (MgO, sapphire, quartz, and Si) have been investigated by 2.7 MeV 4He+ Rutherford backscattering spectrometry. The diffusivities of Cu in the substrates and of the substrate elements (Mg, Al, and Si) in the Y‐Ba‐Cu‐O thin films have been determined over the annealing temperature range from 876 to 1226 K in an oxygen atmosphere. Among three metallic elements of Y, Ba, and Cu, Cu diffuses the fastest into the substrates. Copper silicide is observed in the interfacial region between the film and the Si substrate. Among these substrates, MgO is suggested to be the most stable on annealing.