PIN photodiodes are often used in optical integrated circuits. Although they can feature a very good RF-performance, this can be effected by the optical power density of the incident light. The influence of this effect on the RF-performance of PIN photodiodes is described. When a critical optical power density in the epi-layer is exceeded the 3dB frequencies are cut off. An analytical equation is derived to describe the effect. The results are compared to RF measurements and verified by numerical simulation.