This study focuses on the influence of zirconia on microwave (MW) sintering of zirconia-toughened alumina (ZTA). ZTA samples containing various zirconia powders (unstabilized and stabilized with yttria or ceria) were sintered in a MW cavity, in a sintering cell with or without susceptor. 8 mol% yttria-stabilized zirconia was denser when it was sintered without susceptor. The densification curves also showed different behaviours in the intermediate stage of sintering, between the four samples. The samples with 3 and 8 mol% yttria-stabilized zirconia were easier to densify than the samples with unstabilized and 10 mol% ceria-stabilized zirconia samples. This was confirmed when the samples were heated in a single-mode cavity. These differences of behaviour under MW can be linked to the chemical composition of zirconia. The substitution of zirconium cation by yttrium in the lattice generates an excess of oxygen vacancies and of dipoles. The latter are involved in the MW/material interactions.