We have observed photon‐assisted tunneling effects at 246 and 604 GHz in small‐area superconductor‐insulator‐superconductor (SIS) and superconductor‐insulator‐normal metal (SIN) superconducting junctions. We find excellent agreement with the Tien–Gordon theory, and infer that the responsivity of SIN junctions approaches the quantum limit at low temperatures for photon energies less than 2&Dgr;.