Abstract: In a method for ascertaining temperatures (tqm, tqh, tom, tok) in an oil-cooled transformer, the transformer terminal voltages (2), the winding currents (3) and the ambient temperature (6) are measured. Furthermore, the status (5) of fans and pumps and the switch position (4) of a stepping switch are established. The measured and established variables (2, 3, 4, 5, 6) are fed to a thermohydraulic model (7), in which state variables (19) are calculated with auxiliary variables (9), which are losses (10) in the transformer, heat transfer parameters (11), flow resistances (13) and the oil flow (12), and a hydraulic network of the oil circuit, which has branches and nodes. The state variables (19) are the average temperatures (tqm) and the hotspot temperatures (tqh) in loss-producing parts of the transformer and the average oil temperatures in branches (tom) and in nodes (tok) of the hydraulic network of the oil circuit.