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This study attempts to define the properties of hyaloclastite formations which control their petrophysical characteristics during their progressive alteration. It is based on 140 tuffaceous cores from last glaciation to 2?3 m y. The water content shows a progressive increase with alteration to about 12%, which mainly is bound in the smectite and zeolite alteration minerals. It diminishes when altered to chlorite and disappearance of zeolites indicating the dehydration of the alteration minerals. CO2 content appears to increase stepwise, firstly at about 30%, secondly at 65% and thirdly near 100% alteration. The rock oxidation increases concomitantly with the water content in the rock. This is reflected in the grain density which also diminishes with increasing alteration but increases again when entering into the chlorite-epidote alteration. Changes in porosity show that larger pores (macro porosity) are filled as alteration proceeds, while secondary micro porosity increases, mainly at palagonite/glass margins and not the least within the smectite clays. Micro porosity in fully altered tuffs ranges from about 7?35%. This type of porosity may explain the ?aquiclude character? of hyaloclastites. Permeability ranges from 0.1 to about 12000 mD and a close correlation is shown between porosity and permeability. This correlation, however, is stronger between macro porosity and permeability than the micro porosity. Chemical analysis of fresh glass and palagonite shows the notable leaching of Na2O in the latter and to some extent the CaO, while FeO and TiO2 are slightly enriched. Other oxides do not show obvious tendencies for mobility. The assessment of chemical mobility during alteration, taking into account the filling of macro pores, was unsuccessful, and may be due to the formation of secondary micro porosity which is not the case in alteration of crystallized rocks. A geochemical comparison of the variably altered tuffs with the compositional field of equivalent fresh rocks indicate the limited mobility of the chemical components, except for CaO, Na2O and Rb which show clear indication of being removed.