Abstract: Clinopyroxene solid solutions crystallized at P_tot = 2 kbar in the natural system phonolitic tephrite-H₂O (lava type from Etna eruption 1971) indicate a linear increase of ln Xcatscpx with H₂O^liq %. After coupling this with the In Xcatscpx pressure dependence found through experiments, a simple relation has been set up for clinopyroxene- and plagioclase-bearing magmas. This relation allows us to calculate the amount of water dissolved in the melt, at any known pressure in the crustal range.