The formation constants of CaUO₂(CO₃)₃²⁻ and Ca₂UO₂(CO₃)₃(aq) were determined in NaCl medium at ionic strengths between 0.1 and 1 mol kg_w⁻¹ using time-resolved laser-induced luminescence spectroscopy (TRLS). Spectroluminescence titration of UO₂(CO₃)₃⁴⁻ complex by Ca²⁺ were conducted at atmospheric CO₂(g) and varying pH values in order to eliminate the eventual precipitation of both schoepite (UO₃ : 2H₂O) and calcite (CaCO₃) in aqueous solutions. To identify the stoichiometry of calcium, the slope analyses corrected by the Ringböm coefficient for UO₂(CO₃)₃⁴⁻ relative to pH and CO₂(g)—instead of typical expression relative to UO₂²⁺ and CO₃²⁻—was applied in this work. Satisfactory linear fits assessed the conditional stepwise formation constants in the range of ionic strength employed in this work, the values of which are in good agreement with literature data at comparable ionic strengths. Extrapolations to infinite dilution were realized in the framework of the specific ion interaction theory (SIT), also providing the evaluation of the specific ion interaction coefficients. The cumulative stability constants at infinite dilution was determined to be log₁₀ β°(CaUO₂(CO₃)₃²⁻) = 27.20 ± 0.04 and log₁₀ β°(Ca₂UO₂(CO₃)₃(aq)) = 30.49 ± 0.05, which are in good agreement with extrapolation proposed elsewhere in literature using a different extrapolation framework. The specific ion interaction coefficients were found to be ε(CaUO₂(CO₃)₃²⁻,Na⁺) = (0.29 ± 0.11) and ε(Ca₂UO₂(CO₃)₃(aq),NaCl) = (0.66 ± 0.12) kg_w mol⁻¹. Integration of alkali metals into the ternary species may explain these positive and relatively large interaction coefficients. Implications on the speciation of uranium in clay groundwaters, representative of radioactive waste repositories, and in seawater are discussed.