Solid mixtures of kaolinite-chlorite (+ or -quartz) and/or chlorite-gibbsite have been equilibrated in aqueous 1.0 M NaCl, 0.001 and 0.005 M MgCl 2 solutions at 25, 125 and 175 degrees C and saturated vapor pressures. Reversibility of fluid-mineral equilibria was demonstrated by approaching final solution compositions from high and low log a (super 1/2) Mg (super 2+) /a H (super +) and from silica under- and super-saturation. At the end of the experiments, the solutions were separated using immiscible displacement techniques. The chlorite-kaolinite equilibrium suggests a temperature-dependent reaction mechanism which may partly reflect changes in speciation of aqueous ions as well as variations in reaction kinetics. Log K for the chlorite-kaolinite reactions have been calculated to be 52.43+ or -7.50, 62.33+ or -6.89 and 79.06+ or -2.60 at 25, 125 and 175 degrees C, respectively. A comparison of the measured Mg/H activity ratios at 25 degrees C with values predicted for (end-member) clinochlore-kaolinite equilibria indicates that the experimentally measured assemblage is more stable; this suggests a large value of Gibbs energy of mixing (Delta G omix ) for chlorite solid solutions.