Abstract The influence of two middle chain methylbranched fatty acids and propylene glycol on the penetration of the highly lipophilic model substance pyrene butyric acid into human skin ex vivo has been investigated. The results obtained were compared with experimental data using oleic acid as standard enhancer. Analogously, liberation studies of pyrene butyric acid into artificial lipid acceptor membranes were performed to evaluate the contribution of vehicle effects to the skin penetration results. It was shown that the fatty acids initially improve the liberation of the model substance which corresponds to an increase in skin penetration. When the penetration process was assessed, the dermal concentration profiles of pyrene butyric acid and propylene glycol were almost the same, strongly indicating a cotransport for the lipophilic model substance. Levels of both pyrene butyric acid and propylene glycol did increase when the vehicles contained fatty acid. Furthermore, there is some indication of a more specific action of oleic acid within the stratum corneum. However, the major effect on the penetration of pyrene butyric acid arises from propylene glycol. It is caused by solvent properties and solvent drag or favoured partition, respectively, into the stratum corneum and the hydrophilic epidermis and dermis which are supposed to be the main diffusion barrier for the model penetrant.