Highly porous Ce0.8Sm0.2O1.9 (CSO) was prepared by acid leaching MgO from CSO-MgO (CSM) ceramics with acetic acid solution. The effects of leaching conditions and MgO content were studied. Using 60 vol% acetic acid at 85 degrees C for 8.5 h, the MgO in CSM ceramics was effectively leached out but no CSO was dissolved; thus highly porous CSO with an open porosity of 68% was successfully prepared. The open porosity of sufficiently leached pellet increased with the MgO content, but when the vol% of MgO exceeded 60%, the flexural strength of the leached pellets decreased from 25 to 1 MPa. The leaching approach allows the fabrication of porous CSO with the vol% of MgO up to 60%.