Synthetic strontianite-aragonite solid-solution minerals were dissolved in CO₂-saturated non-stoichiometric solutions of Sr(HCO₃)₂ and Ca(HCO₃)₂ at 25°C. The results show that none of the dissolution reactions reach thermodynamic equilibrium. Congruent dissolution in Ca(HCO₃)₂ solutions either attains or closely approaches stoichiometric saturation with respect to the dissolving solid. In Sr(HCO₃)₂ solutions the reactions usually become incongruent, precipitating a Sr-rich phase before reaching stoichiometric saturation. Dissolution of mechanical mixtures of solids approaches stoichiometric saturation with respect to the least stable solid in the mixture. Surface uptake from subsaturated bulk solutions was observed in the initial minutes of dissolution. This surficial phase is 0–10 atomic layers thick in Sr(HCO₃)₂ solutions and 0–4 layers thick in Ca(HCO₃)₂ solutions, and subsequently dissolves and/or recrystallizes, usually within 6 min of reaction. The initial transient surface precipitation (recrystallization) process is followed by congruent dissolution of the original solid which proceeds to stoichiometric saturation, or until the precipitation of a more stable Sr-rich solid. The compositions of secondary precipitates do not correspond to thermodynamic equilibrium or stoichiometric saturation states. X-ray photoelectron spectroscopy (XPS) measurements indicate the formation of solid solutions on surfaces of aragonite and strontianite single crystals immersed in Sr(HCO₃)₂ and Ca(HCO₃)₂ solutions, respectively. In Sr(HCO₃)₂ solutions, the XPS signal from the outer ~ 60 Å on aragonite indicates a composition of 16 mol% SrCO₃ after only 2 min of contact, and 14–18 mol% SrCO₃ after 3 weeks of contact. The strontianite surface averages approximately 22 mol% CaCO₃ after 2 min of contact with Ca(HCO₃)₂ solution, and is 34–39 mol% CaCO₃ after 3 weeks of contact. XPS analysis suggests the surface composition is zoned with somewhat greater enrichment in the outer ~25 Å (as much as 26 mol% SrCO₃ on aragonite and 44 mol% CaCO₃ on strontianite). The results indicate rapid formation of a solid-solution surface phase from subsaturated aqueous solutions. The surface phase continually adjusts in composition in response to changes in composition of the bulk fluid as net dissolution proceeds. Dissolution rates of the endmembers are greatly reduced in nonstoichiometric solutions relative to dissolution rates observed in stoichiometric solutions. All solids dissolve more slowly in solutions spiked with the least soluble component ((Sr(HCO₃)₂)) than in solutions spiked with the more soluble component (Ca(HCO₃)₂), an effect that becomes increasingly significant as stoichiometric saturation is approached. It is proposed that the formation of a non-stoichiometric surface reactive zone significantly decreases dissolution rates.