This paper investigates the impact of propagation modes on radiated immunity characterization inside most commonly used transverse electromagnetic (TEM) cells. The first six modes together with their corresponding cutoff frequencies in two standard open and closed cells are firstly introduced and presented. It is then followed by an immunity testing case study of the electromagnetic field behavior inside the cells at frequencies below and above the dominant mode. Results imply that, although not recommended in the IEC 61967-2 and 62132-2 immunity testing standards, it is actually feasible to use a TEM cell at frequencies above the dominant mode frequency if the equivalent ‖E‖-field level with respect to the input power is properly rescaled in close proximity to the IC pins whatever the location of the IC package. Moreover, due to the dependency of the location of norm maxima ‖E‖ and ‖H‖ to frequency, attention must be paid to the DUT location in the TEM cell for proper immunity assessment. This calls for the use of an open TEM cell for immunity testing as relocating the DUT position is impossible in the closed cell.