The effects of B2O3 and SiO2 which are major constituents of vitreous low-firing agents on the dielectric properties and reliability of ceramic dielectrics were elucidated for the case of the lead-based relaxor dielectric ceramic [(Pb0.875Ba0.125)][Mg1/3Nb2/3)0.5(Zn1/3Nb2/3)(0.3)Ti-0.2]O-3. Boron oxide (B2O3) led to a decrease in dielectric constant and degraded reliability under a humidity load condition at 85 degrees C and 95% RH. Although SiO2 also caused a decrease in dielectric constant, it did not result in a reliability degradation. Analysis of the microstructure using TEM and STEM revealed that a water-soluble secondary phase consisting of B2O3 and PbO was present at the grain boundaries and triple points in specimens with added B2O3. In contrast, no secondary phase existed in SiO2-doped specimens, but Si segregated at the grain boundaries. The existence of a water-soluble grain boundary phase was shown to degrade lifetime under humidity load conditions.