The effect of an external electrical field on phase behaviors is reported for polymer dispersed liquid crystal films of 4 '-pentyl-4-biphenylcarbonitrile/poly(methyl methacrylate) binary mixtures with various polymer molecular weights. The experimental results show that increasing the molecular weight of the polymer or the electrical field intensity can give rise to an increase in the phase-transition temperature and a widening of the binary phase region. The lattice theory, regarding a binary system consisting of a rigid nematic liquid crystal and a random polymer, has been extended to the case in which an external electrical field is present. A comparison of the theoretical predictions with the experimental results has been carried out, and satisfactory agreement has been found. (c) 2007 Wiley Periodicals, Inc.