Laboratory studies of vacuum dehydration of Mg-O-H units in minerals and rocks reveal large kinetic isotope effects in oxygen, with light-isotope enrichment in the liberated water, and concomitant heavy-isotope enrichment in the residual solids. Application of the same techniques to a meteorite assemblage including hydrous magnesium silicates shows a much more complex behavior, probably due to isotopic heterogeneity inherited from processes on the meteorite parent body. Measurements of both the O-17/O-16 variations and the O-18/O-16 variations confirm theoretical predictions that slopes of three-isotope correlation lines are systematically larger for equilibrium isotope effects than for kinetic isotope effects.