Abstract: The method is to determine a leak in a breast wall of a regenerative heat recuperator having a combustion chamber and a heat storage chamber containing a heat storage medium adjacent to the combustion chamber and separated therefrom by a breast wall. The combustion chamber has inlet means for fuel and air and the heat storage chamber has outlet means for the flue gases which pass into it from the combustion chamber. To achieve a simple and accurate determination, the method comprises the steps of (i) while passing gas through the recuperator, introducing to the inlet means a marker gas which undergoes an irreversible change at a temperature prevailing in the recuperator, and (ii) detecting the presence of the marker gas in the gases passing out of the outlet means.