The influence of optical radiation on the MOSFET threshold voltage has been investigated theoretically as well as experimentally. For conventional MOSFETs the influence is negligible, but for open-gate FET-based sensors, such as the ISFET, optical radiation can cause a considerable threshold voltage shift. An explanation of the threshold voltage shift due to illumination is given, based on the analysis of quasi-equilibrium effects in an illuminated semiconductor surface layer. The relation between the threshold voltage and the optical radiation intensity has been derived. Experimental result are given.