We have examined the effect of submonolayer coverage of 1-pyrene butyric acid on charge carrier transport in reduced graphene oxide. We have modeled the interaction of 1-pyrene butyric acid molecules with graphene and determined the amount of charge transfer at the interface between the two materials. The effect of 1-pyrene butyric acid as electron acceptor was determined by transfer characteristics measurements on thin film transistors for thick layers. By using time-resolved photocurrent measurements we were able to detect a reduction of electron mobility in reduced graphene oxide for coverage as low as 0.08%.