A microwave oven and method for heating a load using microwaves are provided. The microwave oven comprises a cavity adapted to receive a load. The cavity is designed to support at least two predefined mode fields. For each predefined mode field, a resonance frequency in the cavity is known. Further, the microwave oven comprises a frequency-controllable microwave source for feeding microwaves into the cavity via at least one feeding port. The method comprises the steps of measuring a signal reflected from the cavity as a function of the operating frequency of the microwave source and identifying resonance frequencies in the cavity based on the measured signal. Further, the method comprises the steps of selecting, for the predefined modes, at least two of the identified resonance frequencies based on the known resonance frequencies and switching the operating frequency of the microwave source using the selected frequencies. The present invention is advantageous in that it enables uniform heating of the load in the cavity.