A one-way valve which can be utilized in a toilet, particularly in a urinal, to prevent return flow of malodorous gases from the sewage system into the room wherein the toilet is installed has a body which is confined in the toilet. The body carries two sheet- or panel-shaped valving elements at least one of which is resilient to yield to the flow of urine but normally sealingly engages the other element to prevent the flow of gases from the sewage system counter to the direction of flow of the urine. At least one marginal portion of the at least one valving element is free to move away from the corresponding marginal portion of the other element to also permit lateral outflow of urine when the one element is at least partially out of contact with the other element.