The water content in modified spinel, synthesized at 15.5 GPa under hydrous conditions, has been measured by means of secondary ion mass spectrometry (SIMS). We found that the modified spinel crystals contain up to 3.1¿0.4 wt % H2O, which is consistent with the amounts estimated from the deficits in the oxide totals of the microprobe analysis. X-ray diffraction analyses for a single crystal showed that the sample containing 2.5¿0.3 wt % H2O is of the modified spinel structure with the lattice parameters a=5.663(1), b=11.546(2), c=8.247(4) ¿ and V=539.2(5) ¿3. The present results suggest that a considerable amount of H2O may exist as hydrous modified spinel in the mantle transition zone, which could have important implications for the constitution and dynamics of the mantle. ¿ American Geophysical Union 1995