The blueschist-greenschist facies transition for a model basaltic system Na*l2*sO-CaO-MgO-Al*l2*sO*l3*s-SiO*l2*s-H*l2*sO is defined by a univariant reaction: 6 clinozoisite + 25 glaucophane + 7 quartz + 14 H*l2*sO = 6 tremolite + 9 chlorite + 50 albite; for the Fe*l2*sO*l3*s-saturated basaltic system, by a discontinuous one: 4 epidote + 5 Mg-riebeckite + chlorite + 7 quartz = 7 hematite (magnetite) + 4 tremolite + 10 albite + 7 H*l2*sO. These two reactions were experimentally investigated to determine the nature of the blueschist-greenschist tradition. The results have located the first reaction at 350 ± 10°C, 7.8 ± 0.2 Kb and 450 ± 10°C, 8.2 ± 0.4 Kb. Reconnaissance experiments for the second reaction indicate that the minimum pressure for the occurance of epidote + Mg-riebeckite + chlorite + quartz is about 4 Kb at 300°C for f*lO2*s defined by the hematite-magnetite buffer.