The success of the ultrasonic pulse‐echo technique for measurement of the depth of burn necrosis in porcine skin [Med. Phys. 4, 259–263 (1977)] has led to the present investigation of the specific acoustic impedance difference between the necrotic (burn) tissue and the underlying viable (nonburn) tissue. Experimental results show that the ultrasonic group velocities and mass densities are approximately the same in these tissues yielding average values of (1.72±0.12) ×105cm/s for group velocity and (1.093×0.009) g/cm3for the density. The characteristic impedance (density times velocity) differs by at most 3% between necrotic and viable tissues. Measurements of acoustic attenuation show a difference of approximately 70% between these tissues. porcine skin is due primarily to the difference in acoustic attenuation.