A new theory is presented of the photoelectric phenomena, which take place in UV illuminated MOS structures, in the presence of weak electric fields (\E \ < 10(4) V/cm) in the dielectric. The problem is formulated in appropriate equations and solved separately for the cases of zero (J = 0) and nonzero (J 0) photocurrents. The solutions obtained are shown to remain in excellent agreement with experiment, which supports the validity of the theory. This theory finds applications in developing new, very sensitive, photoelectric measurement methods of the MOS system parameters. Two of such methods are shortly presented. The first is the measurement method of the phi (MS) factor of the MOS system, which has already been fully verified experimentally and has been shown to be the most accurate of the existing methods of this parameter determination. The second is the method to determine trapping properties of the dielectric in the MOS system, which is currently being optimized and verified experimentally.