A method for ascertaining a fill level of at least one exhaust-gas component, which can be stored in a catalytic converter and which is generated in a combustion process, in the catalytic converter, wherein a variation of the fill level of the at least one exhaust-gas component in the catalytic converter during the combustion process is determined, wherein, during time periods in which the combustion process is not operated, a diffusion-induced change of the fill level of the at least one exhaust-gas component in the catalytic converter is determined, and wherein, on the basis of the determined variation during the combustion process and the diffusion-induced change, a fill level of the at least one exhaust-gas component in the catalytic converter is ascertained. The invention furthermore relates to a processing unit and to a computer program for carrying out such a method.