A cap closes a sample vessel for the microwave treatment of samples. The cap contains a closure body for closing the sample vessel. The closure body is fitted to the sample vessel and closes the sample vessel at a sealing surface. A spring-loaded pressure relief valve and a vent duct are provided. The vent duct, the pressure relief valve and the closure body are configured in such a manner that the vent duct connects the closure body via the pressure relief valve to the surrounding area such that when a defined first pressure level is exceeded at the closure body excess pressure can escape through the vent duct into the area surrounding the cap. A reservoir is provided. The reservoir and the vent duct are configured such that condensate precipitating in the vent duct accumulates in the reservoir when the cap is in the state fitted to the sample vessel.