A method and device for determining when a food (9) has thawed in a microwave oven (1) are provided. The food (9) is placed above an element (14) constructed of a microwave sensitive material and shields the element (14) from the microwaves to a different extent according to its degree of thawing. The temperature increase in the material resulting from its absorption of the microwaves is measured by a transducer (18) which receives a signal operationally related to the temperature and which on the basis for the signal halts the thawing when the temperature has reached a suitable value. The element (14) is associated with a food support (8) which thermally isolates it from the food (9), the element being in direct or indirect contact with the transducer (18). The transducer (18) can be disposed either in the food support (8) or in the bottom wall (7) of the oven thawing chamber (4) below the support (8).