A method is provided for diagnosing the operability of fuel vapor intermediate stores (4), in particular of activated carbon filters, in tank-venting systems with at least one tank (3) and at least one fuel vapor intermediate store (4). In this case an actual change degree of filling of the fuel vapor intermediate store (4) is determined as a consequence of absorption or desorption processes of gaseous hydrocarbons in the fuel vapor intermediate store (4). The actual change in degree of filling is compared with a desired change in degree of filling and, from the comparison, a conclusion about the operability of the fuel vapor intermediate store (4) is drawn.