Spectrophotometric measurements of a commonly used agarose-based ultrasonic tissue-mimicking gel are reported. In addition, spectral characteristics after heating to different temperatures provide thermostability information. Thermostability of agarose gels has not been previously reported, except in general terms. Gels were produced and cast into 2 mm thick spectrophotometric cuvettes. UV-Vis spectra were recorded after gels were heated to various temperatures. Spectral changes resulting from heating were noted and were largely unchanged with heating to temperatures ranging from 30 to 70 °C. Only with heating to 80 °C, which is near the melting point of the gel material, do spectra irreversibly change. We show that agarose-based tissue-mimicking gels are largely stable over temperatures relevant to most biomedical ultrasonic studies, including those examining hyperthermia or high-intensity focused ultrasound. Therefore, they likely provide a stable substrate in which to conduct ultrasonic heating studies. In addition, UV-Vis spectrophotometry of this optically opaque material has been demonstrated. Possible additives to this material that would enable the measurement of temperature fields through postexposure sectioning and slicing are subsequently discussed.