The possibility of enhancing selectivities and yields in networks of parallel and series reactions is investigated theoretically. Isothermal tubular reactors are considered where reactants can be introduced at the entrance and also over the wall. The latter way of dosing Could be realised, e.g., in a membrane reactor where one or several reactants can be dosed through a porous reactor wall. Besides numerical solutions of the underlying mass balance equations of simplified reactor models, instructive analytical Solutions were derived which are valid under certain constraints. Using these solutions an optimisation of the reactor performance could be performed. As objective function the molar fraction of a desired intermediate product at the reactor outlet was maximised. The impact of influencing via the dosing strategy applied the local composition (and thus the local reaction rates) and the component residence time distributions is elucidated. (C) 2003 Elsevier Ltd. All rights reserved.