To ascertain a parameter of a transformer (40) that has a high voltage side (41) and a low voltage side (43), a test signal generated by a source (13) is impressed on the low voltage side (43). A test response of the transformer (40) is recorded. A leakage reactance and/or a leakage inductance of the transformer (40) is determined by an evaluation device (18) of an apparatus (10) on the basis of the test response of the transformer (40).