The invention relates to a closure for an electrochemical reaction vessel, in particular a potentiostat, the closure comprising: a holder for holding electrodes arranged at an inner side of the closure such that, when the closure is attached to a reaction vessel, electrodes held by the holder extend into an interior space of the reaction vessel and into an electrolyte contained in the reaction vessel; and a plurality of contacts arranged at an outer side of the closure for providing electrical contacts with the electrodes.