We present the first experimental evidence for the ternary complexation of calcium and magnesium ions with plutonyl(vi)tricarbonate species in carbonate-containing aqueous solutions using visible-NIR spectrophotometric titration. Prior to studying the ternary plutonyl(vi) carbonate complexation, visible-NIR absorption spectral information of PuO2(CO3)22- and PuO2(CO3)34- was successfully obtained. PuO2(CO3)22- has a prominent peak at 853 nm and its molar absorptivity was determined to be ε853, PuO2(CO3)22- = 49.0 ± 4.2 M-1·cm-1. The spectrophotometric titration results by adding calcium or magnesium to the plutonyl(vi) carbonate system consisting of PuO2(CO3)22- and PuO2(CO3)34- indicate the formation of CaPuO2(CO3)32- and MgPuO2(CO3)32- complexes and provide the formation constants at 0.1 M H/NaClO4 for MPuO2(CO3)32- from PuO2(CO3)34-, log K = 4.33 ± 0.50 and 2.58 ± 0.18 for M = Ca2+ and Mg2+, respectively. In addition, the formation constants of CaPuO2(CO3)32- and MgPuO2(CO3)32- from PuO2(CO3)34- at infinite dilution (log K°) were proposed to be 6.05 ± 0.50 and 4.29 ± 0.18, respectively, based on the correction of ionic strength using the Davies equation. The absorption spectrum of the ternary plutonyl(vi) complexes of CaPuO2(CO3)32- is similar to that of PuO2(CO3)34- with the exception of a characteristic absorption peak at 808 nm (ε808, CaPuO2(CO3)32- = 42.9 ± 1.6 M-1·cm-1). According to the calculated aqueous plutonyl(vi) speciation including the ternary plutonyl(vi) complexes, CaPuO2(CO3)32- is considered the dominant Pu(vi) species under environmental conditions, and plutonyl(vi) may be more mobile than expected in previous assessments.