The transition properties of thin superconducting tin films are determined through measurements on a new device, the superconductive radio‐frequency mixer. An equivalent circuit for this device is presented which permits the prediction of mixer performance using static transmission curves for the particular films employed. Experimental transmission curves and their characteristics are discussed. The measured mixer performance is compared with theoretical performance for operation in the frequency range of 240 to 840 Mc. It is shown that the transition speed of the superconducting thin films can be determined and that film‐switching times are at least as short as 0.625 nsec.