Method and apparatus for measuring catalytic reactivity in diagnosing a gas-permeable object, such as a catalyst or a filter, that has been removed from an exhaust gas tract connected downstream of an internal combustion engine of, motor vehicle. The method includes applying, through an opening in a device, a test medium with a defined composition, to a front face of the removed object; and measuring, at a position located, with respect to a flow direction of the test medium, downstream of the front face, at least of: a concentration of at least one reduced or oxidized constituent of the test medium after passage of the test medium through the gas-permeable object; and a temperature of the object after the test medium is applied to the gas-permeable object.