Localized-state distributions have been studied in a molecularly doped polymer (MDP) system of a polymer binder (polycarbonate) doped with charge-transporting [N,N′-diphenyl-N, N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD)] and trap-forming molecules [1-phenyl-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)pyrazoline (PRA)] simultaneously by means of the conventional time-of-flight (TOF) transient photocurrent measurements. The existence of a transport energy in the MDP system is experimentally shown by comparing Gaussian distributions of localized states deduced by the Gaussian disorder model, due to Bässler and co-workers [H. Bässler, Phys. Status Solidi B 175, 15 (1993)], with localized-state distributions determined from the analysis of the TOF transient photocurrent data, based on the trap-controlled band transport [H. Naito, J. Ding, and M. Okuda, Appl. Phys. Lett. 64, 1830 (1994)]. The transport energy is found to be located at the center of the Gaussian distribution due to the host TPD molecules. It is also found that at 0.1 mol % PRA addition, the Gaussian distribution of localized states due to TPD molecules is broadened through the random electrostatic potential generated by dipoles of PRA, and at 1 mol % PRA addition, the localized-state structure due to PRA molecules, as well as the further broadening of the Gaussian distribution, are observed. The energy level of the structure is determined to be 0.54 eV above the transport energy, which is almost equal to the difference in the ionization potential between PRA and TPD. © 2000 American Institute of Physics.