A new method of scavenging highly resistive siliceous phase using two-stage sintering, named as "precursor scavenging," is suggested for improving the grain-boundary conductivity of 8 mol % yttria-stabilized zirconia (YSZ). The scavenging efficiency and mechanism were studied and compared with those of 8YSZ-Al2O3 composites prepared by various methods using impedance spectroscopy and imaging secondary-ion mass spectroscopy. A heat-treatment at 1200 degrees C for longer than 20 h before sintering increased grain-boundary conductivity remarkably. The forming of inclusions containing Si was considered to be the origin of scavenging. The grain-interior resistivity was not changed by precursor scavenging, while it increased more than 15% by adding 1 mol % Al2O3 when sintered at 1600 degrees C. Precursor scavenging, therefore, is a potential and promising way for improving the grain boundary conductivity without deteriorating the grain-interior one.