Addition of 0.5mol% of CoO into SnO 2 promotes densification of this oxide to 99% of the theoretical density during sintering. TEM in this system reveals that after sintering at 1210 o C a secondary phase of Co 2 SnO 4 is precipitated at the SnO 2 grain boundaries during cooling. This phase is formed by diffusion of Co ions from the bulk to the grain boundary during sintering leaving needle-like defects at the grain bulk. The high resolution TEM micrograph of this system sintered at 1210 o C and 1400 o C showed an amorphous grain boundary region low in cobalt, indicating that the Co 2 SnO 4 phase is precipitated from this region.