The water content of synthetic amphibole (richterite50 +tremolite50 solid solution) markedly increases with increase in the vapor pressure in the range from 5 to 20 kb at fixed temperatures. Temperature change from 750¿C to 900¿C at fixed pressures does not affect the water content of synthetic amphiboles. The water content of common hornblende from ultramafic-mafic rocks is found to be always higher than that from granitic rocks. This difference indicates a difference in the confining pressrue of formation of two rock types.