The relationship between interannual global mean sea level change and the Earth's water mass budget is examined between 1993 and 1998 by removing the steric (thermal) component from mean sea level computed with TOPEX/Poseidon (T/P) altimetry. The steric component is calculated from subsurface temperatures measured by expendable bathythermographs and interpolated to a global grid by empirical orthogonal function (EOF) reconstruction. Results indicate that from late-1995 to early-1998, the thermal expansion of sea level was significantly higher than the total sea level change measured by T/P, suggesting that fresh water mass was lost from the ocean. The size of the maximum water mass lost is equivalent to about 18 mm of sea level. An error analysis indicates that this value is significant at the 95% confidence level. Results from numerical models show similar magnitudes of water mass change in the ocean at interannual periods, but at different phases.