An ability to precisely regulate the quantity and location of molecular flux is of value in applications such as nanoscale three-dimensional printing, catalysis and sensor design(1-4). Barrier materials containing pores with molecular dimensions have previously been used to manipulate molecular compositions in the gas phase, but have so far been unable to offer controlled gas transport through individual pores(5-18). Here, we show that gas flux through discrete angstrom-sized pores in monolayer graphene can be detected and then controlled using nanometre-sized gold clusters, which are formed on the surface of the graphene and can migrate and partially block a pore. In samples without gold clusters, we observe stochastic switching of the magnitude of the gas permeance, which we attribute to molecular rearrangements of the pore. Our molecular valves could be used, for example, to develop unique approaches to molecular synthesis that are based on the controllable switching of a molecular gas flux, reminiscent of ion channels in biological cell membranes and solid-state nanopores(19).