Time-of-flight ultrasonic measurements were conducted on a thick hexagonal block of 304 stainless steel irradiated to ∼33 dpa in EBR-II, and the results of ultrasonic-implied void swelling and carbide-induced densification were compared with those obtained by immersion density measurements and TEM observation. The three types of measurement were found to agree rather well with each other. This study confirmed that ultrasonic velocity measurement is a powerful non-destructive technique to measure the through-thickness-average volumetric changes induced by neutrons in thick structural materials.