The transparent conductingpoly(3,4-ethylenedioxythiophene) (PEDOT) is of interest for various optoelectronic device applications. Here, the conductivity stability of PEDOT processed using oxidative chemical-vapor-deposition (oCVD) with FeCl3 as an oxidant is primarily dominated by the change in carrier density when aged in air. To establish the mechanism for the conductivity decrease, the changes in carrier density and carrier mobility of PEDOT films are separately monitored using an AC Hall Effect measurement system. The measured electrical properties reveal that a decrease in carrier density dominates the conductivity decrease during annealing. X-ray diffraction analysis made on the HBr- and MeOH-rinsed PEDOT samples identifies the Fe-related dedoping phase of Fe(OH)(2) and provides the dedoping mechanism. The carrier transport study demonstrates heavily doped oCVD PEDOT with the carrier density higher than similar to 10(20) cm(-3), and in this regime, an increase in carrier density yields lower carrier mobility which shows that the carrier transport is governed by the ionized impurity scattering mechanism due to increased dopant counter-anions. These findings of the mechanisms for PEDOT conductivity decrease and carrier transport behavior may be important to organic optoelectronic device applications that show a strong effect of air-exposure and low-temperature annealing on the device stability and performance.