We have studied acceptor-substituted poly(3-butylthiophene) in an attempt to examine the role of acceptor molecules as intrinsic charge traps under light excitations by measuring the transient photoconductivity response following pulse excitation. The specially synthesized acceptor molecule is a chemically prepared high electron affinity (HEA) monomer, 1-(4-nitrophenyl)-2-( 3-thienyl)ethene. In the copolymers prepared with this acceptor monomer we have observed simultaneous decrease of the sub-nanosecond photoconductivity and increase of the slow component (t > 10 ns) photoconductivity, with increasing HEA group concentration in the substituted poly(butylthiophene)s. The slow component is attributed to a bimolecular recombination process.