A nonlinear mixing effect has been predicted theoretically, and measured using a GaInAs PIN photodiode. The nonlinear effect has been simulated using a small signal finite difference model. The variation of the effect has been measured over the frequency range 1.2–15 GHz for a number of bias voltages from 5 to 20 V, and the effect has been shown to be optoelectronic in origin. A high frequency heterodyne experiment has been performed with the optical and local oscillator signals in the range 10–20 GHz. The difference beat frequency was observed at 500 MHz and 2 GHz. The measured conversion loss was between 27 and 40 dB for a 0 dBm local oscillator signal. The application of the optoelectronic mixing effect to optical fibre frequency domain multiplex systems for the dissemination of frequency standards has been considered.