The hole mobilities of molecularly doped polymer films containing p-diethylaminobenzaldehyde diphenylhydrazone (DEH) are measured before and after systematic UV irradiation. Ultraviolet exposure has been shown to induce a photochemical reaction of the DEH molecule. This reaction is known to reduce dramatically the molecule's capability to transport charge. Presumably the reduction in mobility is associated with the substantial increase (about 1 eV) in the ionization potential of the DEH molecule after photocyclization. We demonstrate that systematic UV irradiation of molecularly doped polymer films containing DEH provides a novel approach for diluting the dopant concentration and effectively increasing the intersite separation between “active” dopant molecules in situ. This photochemical process is exploited to study charge transport in DEH-doped polycarbonate films parametric in UV irradiation time. Transport parameters are determined from transient photocurrent measurements over a range of electric fields and temperatures. Our principal observations are: (1) UV irradiation of DEH-doped polycarbonate films results in a corresponding decrease in the effective hole mobility, and (2) despite the systematic drop in mobility, a consequence of the elimination of active hopping sites, the activation energy for hopping, Δ, and the energetic disorder parameter, σ, remain constant.