Frequency mixing of infrared laser radiation in the tunneling junction of a scanning tunneling microscope (STM) was investigated. The dependence of the mixing signal on the tunneling current, the bias voltage, the power and polarization of the laser radiation, and on the difference frequency was studied using a W–Au STM junction. From the frequency dependence of the signals observed up to 90 MHz, the difference frequency generation is attributed to a thermal process. In addition, the generation of harmonics of the beat signal and mixing with a radio frequency signal is reported. The mixing experiments demonstrate a considerable extension of the frequency range accessible with the STM.