This paper presents a solution chemistry-focused analysis of orthorhombic and cubic tricalcium aluminate (orth-C3A and cub-C3A, respectively) hydration. It is shown that the different solubilities of cub- and orth-C3A influence the bulk aqueous Ca to Al concentration ratio and the C3A/solution interface chemistry. The results are consistent with the bulk solution chemistry controlling orth-C3A dissolution, and with cub-C3A dissolution controlled by the formation of an Al-rich leached layer and adsorption of Ca‑sulfur ion pair complexes onto this layer. The polynaphthalene sulfonate-based admixture used here is identified to modify the solution chemistry and retard cub-C3A dissolution. Strategies to inhibit C3A dissolution in Portland cement are discussed.