ZrO2 samples with various CaO contents were fabricated by hot pressing, whereby CaO was dissolved by and/or reacted with ZrO2 to form a solid solution and/or CaZr4O9, respectively. After a reaction with Ti at 1550°C for 6 h in argon, the interfacial microstructures were characterized using X-ray diffraction and analytical electron microscopy. Experimental results were very different from those found previously in the Y2O3-ZrO2 system. The 5 mol% CaO-ZrO2 sample was relatively stable due to the formation of a thin TiO layer acting as a diffusion barrier phase. However, α-Ti(O), β′-Ti (Zr, O), and/or Ti2ZrO were found in 9 or 17 mol% CaO-ZrO2 due to extensive interdiffusion of Ti, O, and Zr with a much thinner (β′-Ti+α-Ti) layer in 17 mol% CaO-ZrO2 than in 9 mol% CaO-ZrO2. Because CaO was hardly dissolved into Ti, it fully remained in the residual ZrO2, leading to the formation of spherical CaZrO3 in 9 mol% CaO-ZrO2 and columnar CaZrO3 in 17 mol% CaO-ZrO2. In the region far from the original interface, abundant intergranular α-Zr was formed in 5 or 9 mol% CaO-ZrO2. Scattered α-Zr and CaZrO3 were found in 17 mol% CaO-ZrO2 because a high concentration of extrinsic oxygen vacancies, which were created by the substitution of Ca+2 for Zr +4, effectively retarded the reduction of zirconia.