Abstract Principles of equilibrium thermodynamics were applied and found useful in evaluating aspects of Frio diagenesis. Solutionmineral equilibria as predictors of reservoir quality and diagenetic history were tested by comparing formation waters from regions of good and poor reservoir quality, the upper and lower Texas coast, respectively. Comparison among waters from these regions was made on activity diagrams of 16 diagenetic reactions such as calcite = ferroan calcite, kaolinite = chlorite, and Ca-montmorillonite = Na-montmorillonite. Relative position of tested waters, with respect to the stability field of authigenic minerals occluding permeability and porosity, was related to reservoir quality. Solution-mineral equilibria are indicators of reservoir quality; equilibria in hydropressured waters best reflect reservoir quality. Activity indices favoring chlorite and ferroan calcite stability and large log([Ca +2 ] .16 /[Na + ] .33 ) ratios are the best indicators of reservoir quality in deep Frio sandstones. Change in ionic strength, analytical molality, and activity indices is correlated with geopressuring. Variation in mole ratios and activity indices with depth is largest between 8000 and 11,000 ft (2440 and 3355 m), the transition zone between the hydropressured and geopressured intervals. The variation is attributed to more active water-rock interaction, or diagenesis, in the transition zone. Diagenesis in the Frio Formation is a function of temperature, pH, activity, and pressure. Predictions made from solution-mineral equilibria add new insight into relative mineral stabilities and in situ pH and are consistent with the diagenetic sequence developed from petrographic data. Calcite equilibrium favors precipitation of calcite early in the burial history. Two stages of chlorite formation are postulated, one early in the hydropressured interval at the expense of smectite grain coats and another late in the geopressured interval at the expense of kaolinite cement. Chlorite and illite are the stable layer silicates in deep Frio sandstones. Albitization of feldspar is initiated in the hydropressured interval at temperatures less than 100°C.