Morphologic changes that occurred during firing in undoped and sulfate-ion-doped yttria powders were examined in the present study. Clear scanning electron microscopy (SEM) images of uncoated insulators were achieved and charging of electrons was avoided by observing small samples, throughout which most of the electrons of the incident beam penetrated. SEM observation and firing of the samples were repeated several times. Searching the observed areas or particles started at low magnification, with the aid of photographs taken earlier. The sulfate-ion dopant inhibited volume diffusion and/or grain-boundary diffusion, and then particle growth of the sulfate-ion-doped yttria proceeded by surface diffusion or evaporation-condensation along with pore growth, which resulted in collapse of the agglomerates of primary particles. Although most of the other particles exhibited slight pore growth along with particle growth at temperatures as low as 800degreesC, a hardening of the agglomerated particles, because of pore elimination by volume diffusion and/or grain-boundary diffusion, occurred at temperatures >850degreesC.