The reaction stilbite=laumontite+3 quartz+3 H2O was experimentally studied using conventional hydrothermal techniques employing mineral mixtures consisting of reactants and products in 9∶1 and 1∶9 ratios. Equilibrium was demonstrated; the univariant curve passes through about 170° C and 2000 bars, 185°±10° C and 3000 bars, about 185° C and 4000 bars, and 183°±10° C at 5000 bars Pfluid. These results combined with published equilibria for analcime, laumontite, wairakite and prehnite permit delineation of the P-T conditions for the zeolite and prehnite-pumpellyite facies metamorphism in the Tanzawa Mountains, Japan.