The microstructure development in pure and ZnO-, Nb2O5- or (ZnO + Nb2O5)-containing SnO2 has been studied. Sintering of pure SnO2 proceeds by an evaporation-condensation mechanism and grain and pore sizes increase without densification. The pores are open with no closed pores developed. Grain and pore structures become homogeneous with sintering time. Sintered pure SnO2 is porous and has large grains. Nb2O5 suppresses particle coarsening and pore growth but does not promote densification, leading to porous sintered bodies with small grains. ZnO and (ZnO + Nb2O5) promote densification but do not suppress particle and grain coarsening. The sintered body is composed of large grains with pores located on grain boundaries.