The mechanism of hole transport in a polymer matrix doped with various different molecules was studied by means of time-of-flight photoconductivity measurements and molecular-orbital calculations. It was found that the magnitude of the drift mobility of holes in molecularly doped polymers is closely related to electronic properties of dopant molecules. At high molecular concentrations, diagonal and off-diagonal disorders dominating the general feature of charge transport are mainly caused by the spatially inhomogeneous distribution of charge on a dopant molecule