The present work studies the effect of thermal aging and 1 mol % Yb 2 O 3 codoping on the metastable tetragonal phases present in 8−9 mol % Sc 2 O 3 −ZrO 2 . Dense ceramic discs of xYb 2 O 3 −(y − x)Sc 2 O 3 −(100− y)ZrO 2 (x = 0 and 1 mol %; y = 8 and 9 mol %) are prepared using conventional solid-state route. XRD, TEM and Raman spectroscopy analysis confirmed the metastable t′phase in 8Sc 2 O 3 −92ZrO 2 and t″-phase in 9Sc 2 O 3 −91ZrO 2 . Although t′-phase is retained on Yb 2 O 3 codoping in 8Sc 2 O 3 −92ZrO 2 , the tetragonality reduces. However, codoping does not affect the t″-phase present in 9Sc 2 O 3 −91ZrO 2 . After aging at 650 °C for 2000 h in air, a decrease in unit-cell parameters ratio and weak XRD peak-splitting are observed in 8Sc 2 O 3 −92ZrO 2 and 1Yb 2 O 3 −7Sc 2 O 3 − 92ZrO 2 suggesting a reduction in unit-cell tetragonality, though t′-phase still exists in both the compositions. In compositions possessing t″-phase, a slight lattice contraction is detected on thermal aging. The tetragonality induced by oxygen-ion shuffling inside the unit-cell increases after aging in all the compositions. This shuffling is lower in the aged sample of 1Yb 2 O 3 − 7Sc 2 O 3 −92ZrO 2 compared to 8Sc 2 O 3 −92ZrO 2 . However, it remains similar in both 1Yb 2 O 3 −8Sc 2 O 3 −91ZrO 2 and 9Sc 2 O 3 − 91ZrO 2 , suggesting a negligible codoping effect on t″-phase. These results demonstrate the effectiveness of Yb 2 O 3 codoping in lowering the tetragonality of metastable t′-phase, which may positively affect the conductivity of ScSZ.