A method for tightness control of a filling-sealing unit (1) for containers (9) is described. According thereto, a pressure chamber formed at the filling-sealing unit is sealed by way of a first seal around a sealer that is movable therein and by way of a second seal around a pressure supply that is movable therein. According to the invention, a third seal formed for sealing the pressure chamber around a container neck finish is sealed from the outside with an attachment, a positive pressure and/or negative pressure is generated in the pressure chamber, the pressure line used for this purpose is closed, and then a pressure profile in the pressure chamber is measured. This allows a leakage to be selectively detected at the first and/or second seal and be distinguished from a leak at the third seal.