The effect of the sintering atmosphere on the defect chemistry and electrical characteristics of the grain boundaries of polycrystalline SrTi0.99Nb0.01O3 (STNO) ceramics, which were synthesized from semiconducting STNO powders by pressureless sintering, was investigated. The ratio of O/(Sr+Ti) at the grain boundaries in the STNO ceramics varied from 1.6 to 2.1 when the atmosphere changed from nitrogen-hydrogen to air, while the charge density of the excess negative layer near the boundaries varied from 1 to 1.3 C/cm(2). The charge density distribution was correlated with the observed electrical features of the grain boundaries.