A series of poly(1,4-phenylene vinylene) copolymers having different comonomer units were synthesized in free-standing films and doped with I2 or FeCl3. Inclusion of very low levels of comonomer units, regardless of the nature of substituents enhances dopability and, thus, electrical conductivity. Moreover, for all of the copolymer systems, electrical conductivity does not linearly depend on the composition. Instead, either it increases rapidly with the content of the comonomer unit reaching to a maximum value and then decreases afterwards, or initial rapid increase is followed by a slow approach to an asymptotic value.