The effects of TaO_2.5 additions to Y₄Zr₃O₁₂ (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO₂-YO_1.5-TaO_2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO_2.5 for interfacial compatibility. Oxides based on Y₄Zr_3-xTa_xO_12+0.5x (x = =0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300 °C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta⁵⁺ content with the lowest Ca:Si melt. It is found that the addition of Ta⁵⁺ increases the capture of Y³⁺ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y³⁺ and Ta⁵⁺ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y³⁺ that stabilize the cubic form. As the Ca²⁺ content of the melt increases, so does the amount of Ca²⁺ incorporated in fluorite. The latter also interacts attractively with Ta⁵⁺ and leads to a reduction of captured Y³⁺ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y³⁺available in the melt to form apatite in TBC-CMAS interactions.