We present the first experimental results of quenching (ionization) of H(n0=43-49) atoms by a linearly polarized microwave electric field consisting of two frequencies (7.58 and 11.89 GHz). Adding a second frequency increases the quenching probability over that for only one frequency, especially where there is local stability at scaled frequencies near certain rational fractions, e.g., n03=16 and 15. Some, but not all, two-frequency quench curves display a rich structure that appears to be related to that observed in some single-frequency quench curves. © 1988 The American Physical Society.