Equilibrium aK (super +) /aH (super +) ratios for three divariant hydrolysis reactions involving muscovite, K-feldspar, and andalusite in the quartz-present region of the system K 2 O-Al 2 O 3 -SiO 2 -H 2 O were determined in 2 molal KCl-HCl solutions between 400 degrees and 800 degrees C and from one to seven kilobars. The three reactions are:1.5KAlSi 3 O 8 + H (super +) = 0.5KAl 3 Si 3 O 10 (OH) 2 + 3SiO 2 + K (super +) KAl 3 Si 3 O 10 (OH) 2 + H (super +) = 1.5Al 2 SiO 5 + 1.5SiO 2 + 1.5H 2 O + K (super +) KAlSi 3 O 8 + H (super +) = 0.5Al 2 SiO 5 + 2.5SiO 2 + 0.5H 2 O + K (super +) which in closed system can be represented by divariant surfaces in P-T-Sigma K/Sigma H space. The surfaces meet at the univariant curve represented by:2KAl 3 Si 3 O 10 (OH) 2 + 4.5SiO 2 + K (super +) = 3KAlSi 3 O 8 + 1.5Al 2 SiO 5 + 1.5H 2 O + H (super +) Isobaric equilibrium conditions for this reaction at one and two kilobars are:1 Kb, 625 + or - 15 degrees C, log Sigma K/Sigma H = 0.32 Kb, 670 + or - 15 degrees C, log Sigma K/Sigma H = 0.9Activities of species in solution at experimental P and T were calculated from Sigma K/Sigma H data, available ionization constant data, and molal volume data. These calculations suggest that hydrothermal fluids rising from depth, if buffered by the above reactions, are initially neutral, but may become about 3 pH units acid at depths equivalent to one kilobar total pressure. These data are consistant with recently published values of lower pressures and temperatures, and also imply a possible error of about five kilocalories in the free energy of formation of andalusite.