A hydrothermal study was undertaken to determine the stability of endmember illite with respect to muscovite at 100–250°C. Experiments were conducted using Brazilian (BR) muscovite or San Juan (RM30a, SG4a) illite, kaolinite, or microcline and quartz or amorphous silica with an equal weight of 2 N1 KCl/HCl solution from 100–250°C and P v,soln to determine solution compositions in equilibrium with solids. Approach to equilibrium was verified by starting with low a kK+/ a H+-low a H 4 SiO 4 , high a k + / a H + low a H 4 SiO 4 , and high a K + / a H + -high a H 4 SiO 4 solutions. Post-experiment solutions were separated from solids and analyzed for pH, m K + , and M H 4 SiO 4 . Solids were analyzed by XRD in this study; TEM investigations of the solid products of complementary experiments will be reported in a subsequent paper. Experimental data were used to construct isothermal, isobaric log a K + / a H + vs. log a H 4 SiO 4 diagrams 100, 150, 200, and 250°C. These data define univariant lines which represent equilibrium between micalike solubility-controlling phases and kaolinite, microcline, or diaspore. Slopes of univariant lines were used to infer the compositions of solubility-controlling phases: 0.28 ± 0.04, 0.51 ± 0.04, and 0.88 ± 0.04K/O 10(OH) 2. A 0.69K/0 10(OH) 2 solubility-controlling phase may also exist but a solubility-controlling phase with a composition close to that of endmember muscovite [1.OK/O,o(OH)2] (Aja et al., 1991; Aja, 1991) was not observed. Solubility-controlling phases with compositions of 0.28, 0.51, and 0.69K/O 10 (OH) 2 appear to represent discrete, metas,table steps encountered in the formation of the stable phase, endmember illite (0.88K/ O 10(OH) 2). Mechanisms proposed for the conversion of smectite to illite based on natural occurrences fail to account for discrete, illitic solubility-controlling phases. The discrepancy between natural and experimentally-based mechanisms for the conversion of smectite to illite may, in part, reflect the different methods used to characterize I S in these studies. Bulk methods (e.g., XRD) provide a macroscopic view of the phases present in natural samples, whereas solubility experiments may be used to infer the presence of minute quantities of solubility-controlling phases that exist within bulk samples.