An acoustic, pulse-echo technique has been used as part of the large-scale, sodium/concrete interaction tests being conducted at Sandia Laboratories to measure the penetration of a large pool (up to 186 kg) of liquid sodium (initial temperature as high as 600°C) into a limestone concrete crucible. The acoustic data give information on the penetration, penetration rate, and condition of the sodium/concrete interface during the test. Two distinct attack modes have been seen. The first is chemical, and proceeds initially at ∼i mm/min and is of limited extent. The second is mechanical (spallation), and proceeds more rapidly (∼4 mm/min) and appears to be limited only by the reaction kinetics.