Low-temperature co-fired ceramics (LTCCs) that are composed of a RuO2-based resistor and a cordierite-glass substrate have been sintered at temperatures of 850 degrees and 900 degreesC. The microstructure of the resistor/substrate interface has been investigated using scanning and transmission electron microscopy, and its correlation to the overall resistance has been discussed. X-ray diffractometry has revealed that lead ruthenate pyrochlore (Pb2Ru2O6.5) in peak-fired thick-film resistors (TFRs) disappears and the co-fired samples contain only RuO2 in the resistor film when sintered at 900 degreesC, The overall resistance of the LTCC resistors is increased by a factor of similar to3 when temperature is increased from 850 degreesC to 900 degreesC. The cordierite- glass composition of the initial substrate reacts with glass in the resistor film. The greatly extended layer of the resistor/substrate interface that contains the conductor particles is either broad or diffuse, which contrasts the abrupt interface that often is observed in conventional TFRs, This layer contains predominantly faceted platelike crystals of anorthite, in addition to other phases (such as diopside, sapphirine, and cristobalite) that apparently crystallize during co-firing as vitrification and chemical reactions between glass compositions of the substrate and the resistor occur. The increase in the resistance of the LTCC resistors is attributed to the interruption of the conducting path by platelike anorthite crystals that are produced in the resistor/substrate interface when subjected to co-firing.