Ruthenium-based thick-film resistors (TFRs) with lead borosilicate glasses of various compositions have been examined for their electrical properties. A striking increase in resistivity has been observed with lead-depleted glasses, and analysis of this increase reveals that it is derived from the critical loss of Ru ions/clusters in the glass. With high-lead-content glasses, the resistivity decreases by two orders of magnitude, in conjunction with the structural inclination of glass toward PbO . SiO2, 2PbO . SiO2, and 4PbO . B2O3, which are believed to accommodate more Ru ions/clusters. These behaviors substantiate the role of Ru ions/clusters in glass as an indispensable entity for electrical conduction in TFRs.