For application in un‐thermostated delay lines a delay medium possessing a low acoustic loss, a zero temperature coefficient of delay and a zero or low aging rate is required. Present isopaustic glasses meet these requirements satisfactorily with the exception of aging which is found to be dependent upon thermal history to a degree that causes difficulty in some applications. This paper reports a new approach to forming a zero temperature coefficient delay medium consisting of the combination in a hot pressed ceramic, with grain size much less than the acoustic wavelength, of two individually stable materials possessing temperature coefficients opposite in sign. Three mixtures have been prepared with different mass ratios of vitreous SiO2and powdered crystalline MgF2by pressing at temperatures of 900°–1120°C and at pressures of 7000–20 000 psi. A zero temperature coefficient is found at 41°C for a mixture with a mass ratio (SiO2/MgF2) of 0.360, a positive coefficient is found for a mixture with mass ratio of 0.306 and a negative coefficient for a mass ratio of 0.419. The internal friction of the mixtures near 1.5 MHz is comparable to and even lower than that of typical isopaustic glasses, though the frequency dependence of the loss is higher. For the mixture with mass ratio of 0.360 the attenuation coefficient depends on frequency asf2.3. Tests have shown no measurable room temperature aging over an eight week period following a mild heating to 110°C.