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Introduction: Thermocouples located inside conventional ablation catheters do not measure tissue temperature nor determine thermal lesion size during cardiac catheter ablation. Hypothesis: Progressive, thermally mediated changes in tissue composition can be detected by the return loss spectrum of a microwave antenna coupling to tissue in the microwave band. Results: Return loss (RL) exhibited characteristic changes in the 1.6 to 1.7 GHz band during heating (Panel A). Heating to a temperature lethal to the myocardium (50°C) reduced the RL amplitude of the sample from baseline by 346.7% (-10.80 ± 4.430 dB, p = 0.0055). Further temperature rise restituted RL amplitude back to approximately baseline values at 64°C to 70°C (Panel B). Conclusion: The pattern of RL amplitude change in the 1.6 - 1.7 GHz microwave band may be useful for detecting lethal thermal myocardial injury and prevent overheating. This observation may have applications for monitoring cardiac catheter ablation.