Double Electron Muon Resonance (DEMUR) is a double resonance technique for the study of muonium defect centers in insulators and semiconductors. A DEMUR experiment was performed at a low rf frequency to investigate the response of the muonium coupled spin system to perpendicular static and oscillating fields of comparable magnitude. The experiment was performed in fused quartz with a linearly polarized rf field resonant with the .dm =1 transition of the muonium triplet in a five gauss static field. For small rf field amplitudes, the results were as predicted by a theory consistent with the rotating wave approximation. For rf fields comparable in magnitude to the static field, significant deviations from the theory were observed. A theory for DEMUR with large alternating fields has not been developed.