We report measurements of the rate of hydrous alteration of amorphous Mg-SiO smokes as a function of temperature as inferred by observing changes in the infrared spectra of these materials. We show that under the conditions reported for the nucleus of Comet Halley based on measurements made by the Vega and Giotto missions, amorphous, anhydrous Mg-SiO smokes would become hydrated within several weeks in the dusty regolith observed on the surface. However, if such grains were released in ''jets'' or from loose ice fragments, then previously amorphous grains would retain their anhydrous nature. Similarly, brief periods of aqueous activity on meteorite parent bodies would convert amorphous, fine-grained material to hydrated phyllosilicates much more rapidly than coarse mineral grains. A kinetic model might therefore be developed to explain the observed textural relationships in the matrices of carbonaceous chondrites such as Mokoia, where amorphous phyllosillicates are intimately associated with coarse anhydrous grains. ¿ American Geophysical Union 1987